MFC 328608: Export tcp_always_keepalive for use by the Chelsio TOM module.

This used to work by accident with ld.bfd even though always_keepalive
was marked as static. LLD honors static more correctly, so export this
variable properly (including moving it into the tcp_* namespace).

Relative to HEAD the MFC includes two additional changes:
- The t3_tom module used for cxgb(4) is also patched.
- A strong reference from the new name (tcp_always_keepalive) to the old
name (always_keepalive) has been added to preserve the KBI for existing
modules.

Suggested by: kib (strong reference)
Sponsored by: Chelsio Communications

MFC r314952: Fix unused variable when built without INVARIANT_SUPPORT.

MFC r286227, r286443:

r286227:

Decompose TCP INP_INFO lock to increase short-lived TCP connections scalability:

- The existing TCP INP_INFO lock continues to protect the global inpcb list
stability during full list traversal (e.g. tcp_pcblist()).

- A new INP_LIST lock protects inpcb list actual modifications (inp allocation
and free) and inpcb global counters.

It allows to use TCP INP_INFO_RLOCK lock in critical paths (e.g. tcp_input())
and INP_INFO_WLOCK only in occasional operations that walk all connections.

PR: 183659
Differential Revision: https://reviews.freebsd.org/D2599
Reviewed by: jhb, adrian
Tested by: adrian, nitroboost-gmail.com
Sponsored by: Verisign, Inc.

r286443:

Fix a kernel assertion issue introduced with r286227:
Avoid too strict INP_INFO_RLOCK_ASSERT checks due to
tcp_notify() being called from in6_pcbnotify().

Reported by: Larry Rosenman <ler@lerctr.org>
Submitted by: markj, jch

MFC r259897:

In sys/dev/cxgb/ulp/tom/cxgb_cpl_io.c, remove static functions
mk_cpl_barrier_ulp(), mk_get_tcb_ulp() and mk_set_tcb_field_ulp(), which
are all unused since r237263.

MFC r259897:

In sys/dev/cxgb/ulp/tom/cxgb_cpl_io.c, remove static functions
mk_cpl_barrier_ulp(), mk_get_tcb_ulp() and mk_set_tcb_field_ulp(), which
are all unused since r237263.