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 r294840

Persist timers TCPTV_PERSMIN and TCPTV_PERSMAX are hardcoded with 5 seconds and
60 seconds, respectively. Turn them into sysctls that can be tuned live. The
default values of 5 seconds and 60 seconds have been retained.

MFC r284245:
Fix a callout race condition introduced in TCP timers callouts with r281599.
In TCP timer context, it is not enough to check callout_stop() return value
to decide if a callout is still running or not, previous callout_reset()
return values have also to be checked.

MFC: r280904, r280990, r281599

r280904:
Use appropriate timeout_t* instead of void* in tcp_timer_activate()

Suggested by: imp
Differential Revision: https://reviews.freebsd.org/D2154
Reviewed by: imp, jhb
Approved by: jhb

r280990:
Provide better debugging information in tcp_timer_activate() and
tcp_timer_active()

Differential Revision: https://reviews.freebsd.org/D2179
Suggested by: bz
Reviewed by: jhb
Approved by: jhb

r281599:
Fix an old and well-documented use-after-free race condition in
TCP timers:
- Add a reference from tcpcb to its inpcb
- Defer tcpcb deletion until TCP timers have finished

Differential Revision: https://reviews.freebsd.org/D2079
Submitted by: jch, Marc De La Gueronniere <mdelagueronniere@verisign.com>
Reviewed by: imp, rrs, adrian, jhb, bz
Approved by: jhb
Sponsored by: Verisign, Inc.

MFC r264321, r264342, r264351, r264356, r273850, r274629:

Currently, the TCP slow timer can starve TCP input processing while it
walks the list of connections in TIME_WAIT closing expired connections
due to contention on the global TCP pcbinfo lock.

To remediate, introduce a new global lock to protect the list of
connections in TIME_WAIT. Only acquire the TCP pcbinfo lock when
closing an expired connection. This limits the window of time when
TCP input processing is stopped to the amount of time needed to close
a single connection.

Approved by: jhb (mentor)

MFC r284245:
Fix a callout race condition introduced in TCP timers callouts with r281599.
In TCP timer context, it is not enough to check callout_stop() return value
to decide if a callout is still running or not, previous callout_reset()
return values have also to be checked.

MFC: r280904, r280990, r281599

r280904:
Use appropriate timeout_t* instead of void* in tcp_timer_activate()

Suggested by: imp
Differential Revision: https://reviews.freebsd.org/D2154
Reviewed by: imp, jhb
Approved by: jhb

r280990:
Provide better debugging information in tcp_timer_activate() and
tcp_timer_active()

Differential Revision: https://reviews.freebsd.org/D2179
Suggested by: bz
Reviewed by: jhb
Approved by: jhb

r281599:
Fix an old and well-documented use-after-free race condition in
TCP timers:
- Add a reference from tcpcb to its inpcb
- Defer tcpcb deletion until TCP timers have finished

Differential Revision: https://reviews.freebsd.org/D2079
Submitted by: jch, Marc De La Gueronniere <mdelagueronniere@verisign.com>
Reviewed by: imp, rrs, adrian, jhb, bz
Approved by: jhb
Sponsored by: Verisign, Inc.

MFC r264321, r264342, r264351, r264356, r273850, r274629:

Currently, the TCP slow timer can starve TCP input processing while it
walks the list of connections in TIME_WAIT closing expired connections
due to contention on the global TCP pcbinfo lock.

To remediate, introduce a new global lock to protect the list of
connections in TIME_WAIT. Only acquire the TCP pcbinfo lock when
closing an expired connection. This limits the window of time when
TCP input processing is stopped to the amount of time needed to close
a single connection.

Approved by: jhb (mentor)