MFC r332387: Annotate geom modules with MODULE_VERSION

GEOM ELI may double ask the password during boot. Once at loader time, and
once at init time.

This happens due a module loading bug. By default GEOM ELI caches the
password in the kernel, but without the MODULE_VERSION annotation, the
kernel loads over the kernel module, even if the GEOM ELI was compiled into
the kernel. In this case, the newly loaded module
purges/invalidates/overwrites the GEOM ELI's password cache, which causes
the double asking.

This MFC commit differs slightly from head, due to pc98 removal. These
changes were trivial and should be obvious.

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 GEOM direct dispatch changes from the projects/camlock branch.

When safety requirements are met, it allows to avoid passing I/O requests
to GEOM g_up/g_down thread, executing them directly in the caller context.
That allows to avoid CPU bottlenecks in g_up/g_down threads, plus avoid
several context switches per I/O.

The defined now safety requirements are:
- caller should not hold any locks and should be reenterable;
- callee should not depend on GEOM dual-threaded concurency semantics;
- on the way down, if request is unmapped while callee doesn't support it,
the context should be sleepable;
- kernel thread stack usage should be below 50%.

To keep compatibility with GEOM classes not meeting above requirements
new provider and consumer flags added:
- G_CF_DIRECT_SEND -- consumer code meets caller requirements (request);
- G_CF_DIRECT_RECEIVE -- consumer code meets callee requirements (done);
- G_PF_DIRECT_SEND -- provider code meets caller requirements (done);
- G_PF_DIRECT_RECEIVE -- provider code meets callee requirements (request).
Capable GEOM class can set them, allowing direct dispatch in cases where
it is safe. If any of requirements are not met, request is queued to
g_up or g_down thread same as before.

Such GEOM classes were reviewed and updated to support direct dispatch:
CONCAT, DEV, DISK, GATE, MD, MIRROR, MULTIPATH, NOP, PART, RAID, STRIPE,
VFS, ZERO, ZFS::VDEV, ZFS::ZVOL, all classes based on g_slice KPI (LABEL,
MAP, FLASHMAP, etc).

To declare direct completion capability disk(9) KPI got new flag equivalent
to G_PF_DIRECT_SEND -- DISKFLAG_DIRECT_COMPLETION. da(4) and ada(4) disk
drivers got it set now thanks to earlier CAM locking work.

This change more then twice increases peak block storage performance on
systems with manu CPUs, together with earlier CAM locking changes reaching
more then 1 million IOPS (512 byte raw reads from 16 SATA SSDs on 4 HBAs to
256 user-level threads).

Sponsored by: iXsystems, Inc.
MFC after: 2 months

MFprojects/camlock r254895:
Add unmapped BIO support to GEOM ZERO if kern.geom.zero.clear is cleared.

Mark all SYSCTL_NODEs static that have no corresponding SYSCTL_DECLs.

The SYSCTL_NODE macro defines a list that stores all child-elements of
that node. If there's no SYSCTL_DECL macro anywhere else, there's no
reason why it shouldn't be static.

In g_zero_destroy_geom(), return 0 instead of EBUSY in the success case.
EBUSY was probably used as a workaround for the deadlock fixed in r207671.

Approved by: pjd
X-MFC after: r207671

Remove trailing spaces.

Move $FreeBSD$ from comment to __FBSDID().

- Allow to specify the byte which will be used for filling read buffer.
- Improve sysctl description a bit.

Submitted by: Ivan Voras <ivoras@gmail.com>

Add a very simple and small GEOM class - ZERO.
It creates very huge provider (41PB) /dev/gzero.
On BIO_READ request it zero-fills bio_data and on BIO_WRITE it does nothing.
You can also set kern.geom.zero.clear sysctl to 0 to do nothing even for
BIO_READ.

I'm using it for performance testing where it is very helpful.

MFC after: 3 days