MFC r312770 and r337854:

After the in_control() changes in r257692, an existing address is
(intentionally) deleted first and then completely added again (so all the
events, announces and hooks are given a chance to run).

This cause an issue with CARP where the existing CARP data structure is
removed together with the last address for a given VHID, which will cause
a subsequent fail when the address is later re-added.

This change fixes this issue by adding a new flag to keep the CARP data
structure when an address is not being removed.

There was an additional issue with IPv6 CARP addresses, where the CARP data
structure would never be removed after a change and lead to VHIDs which
cannot be destroyed.

PR: 229384
Sponsored by: Rubicon Communications, LLC (Netgate)

Copy head@r302406 to stable/11 as part of the 11.0-RELEASE cycle.
Prune svn:mergeinfo from the new branch, as nothing has been merged
here.

Additional commits post-branch will follow.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Merge 'struct ip6protosw' and 'struct protosw' into one. Now we have
only one protocol switch structure that is shared between ipv4 and ipv6.

Phabric: D476
Reviewed by: jhb

Migrate struct carpstats to PCPU counters.

Add const qualifier to the dst parameter of the ifnet if_output method.

Resolve source address selection in presense of CARP. Add a couple
of helper functions:

- carp_master() - boolean function which is true if an address
is in the MASTER state.
- ifa_preferred() - boolean function that compares two addresses,
and is aware of CARP.

Utilize ifa_preferred() in ifa_ifwithnet().

The previous version of patch also changed source address selection
logic in jails using carp_master(), but we failed to negotiate this part
with Bjoern. May be we will approach this problem again later.

Reported & tested by: Anton Yuzhaninov <citrin citrin.ru>
Sponsored by: Nginx, Inc

Restore a feature that was present in 5.x and 6.x, and was cleared in
7.x, 8.x and 9.x with pf(4) imports: pfsync(4) should suppress CARP
preemption, while it is running its bulk update.

However, reimplement the feature in more elegant manner, that is
partially inspired by newer OpenBSD:

- Rename term "suppression" to "demotion", to match with OpenBSD.
- Keep a global demotion factor, that can be raised by several
conditions, for now these are:
- interface goes down
- carp(4) has problems with ip_output() or ip6_output()
- pfsync performs bulk update
- Unlike in OpenBSD the demotion factor isn't a counter, but
is actual value added to advskew. The adjustment values for
particular error conditions are also configurable, and their
defaults are maximum advskew value, so a single failure bumps
demotion to maximum. This is for POLA compatibility, and should
satisfy most users.
- Demotion factor is a writable sysctl, so user can do
foot shooting, if he desires to.

A major overhaul of the CARP implementation. The ip_carp.c was started
from scratch, copying needed functionality from the old implemenation
on demand, with a thorough review of all code. The main change is that
interface layer has been removed from the CARP. Now redundant addresses
are configured exactly on the interfaces, they run on.

The CARP configuration itself is, as before, configured and read via
SIOCSVH/SIOCGVH ioctls. A new prefix created with SIOCAIFADDR or
SIOCAIFADDR_IN6 may now be configured to a particular virtual host id,
which makes the prefix redundant.

ifconfig(8) semantics has been changed too: now one doesn't need
to clone carpXX interface, he/she should directly configure a vhid
on a Ethernet interface.

To supply vhid data from the kernel to an application the getifaddrs(8)
function had been changed to pass ifam_data with each address. [1]

The new implementation definitely closes all PRs related to carp(4)
being an interface, and may close several others. It also allows
to run a single redundant IP per interface.

Big thanks to Bjoern Zeeb for his help with inet6 part of patch, for
idea on using ifam_data and for several rounds of reviewing!

PR: kern/117000, kern/126945, kern/126714, kern/120130, kern/117448
Reviewed by: bz
Submitted by: bz [1]

Unbreak LINT by moving all carp hooks to net/if.c / netinet/ip_carp.h, with
the appropriate ifdefs.

Reviewed by: bz
Approved by: ken (mentor)

Allow carp(4) to be loaded as a kernel module. Follow precedent set by
bridge(4), lagg(4) etc. and make use of function pointers and
pf_proto_register() to hook carp into the network stack.

Currently, because of the uncertainty about whether the unload path is free
of race condition panics, unloads are disallowed by default. Compiling with
CARPMOD_CAN_UNLOAD in CFLAGS removes this anti foot shooting measure.

This commit requires IP6PROTOSPACER, introduced in r211115.

Reviewed by: bz, simon
Approved by: ken (mentor)
MFC after: 2 weeks

Update stats in struct carpstats using two new macros: CARPSTATS_ADD()
and CARPSTATS_INC(), rather than directly manipulating the fields of
the structure. This will make it easier to change the implementation
of these statistics, such as using per-CPU versions of the data
structure.

MFC after: 3 days

Make sure that carp_header is 36 bytes long

o Introduce carp_multicast_cleanup(), which removes and frees
multicast addresses from carp interface. [1]
o Rewrite carpdetach(), so that it does the following things: [1]
- Stops callouts.
- Decrements carp_suppress_preempt, if needed.
- Downs interface and sets CARP state to INIT.
- Calls carp_multicast_cleanup().
- Detaches softc from carp_if and if we are the last frees
the carp_if.
o Use new carpdetach() in carp_clone_destroy().
o In carp_ifdetach() acquire the carp_if lock and cleanup all
interfaces hanging on carp_if. [1]
o Make carp_ifdetach() static and use EVENT(9) to call it
from if_detach(). [2]
o In carp_setrun() exit if the softc doesn't have a valid pointer
to parent. [1]

Obtained from: OpenBSD [1]
Submitted by: Dan Lukes <dan obluda.cz> [2]
PR: kern/82908 [2]

Add CARP (Common Address Redundancy Protocol), which allows multiple
hosts to share an IP address, providing high availability and load
balancing.

Original work on CARP done by Michael Shalayeff, with many
additions by Marco Pfatschbacher and Ryan McBride.

FreeBSD port done solely by Max Laier.

Patch by: mlaier
Obtained from: OpenBSD (mickey, mcbride)