Fix an infinite loop in tcp_tw_2msl_scan() when an INP_TIMEWAIT inp has been destroyed before its tcptw with INVARIANTS undefined.

[jch]

Summary

Fix an infinite loop in tcp_tw_2msl_scan() when an INP_TIMEWAIT inp has been destroyed before its tcptw wint INVARIANTS undefined.

This is a symmetric change of r307551:

A INP_TIMEWAIT inp should not be destroyed before its tcptw, and INVARIANTS
will catch this case. If INVARIANTS is undefined it will emit a log(LOG_ERR)
and avoid a hard to debug infinite loop in tcp_tw_2msl_scan().

Reported by: Ben Rubson, hselasky
Submitted by: hselasky
Tested by: Ben Rubson, jch
MFC after: 1 week
Sponsored by: Verisign, inc

[mjoras]

For anyone wondering about the history here, see this thread from -net: https://lists.freebsd.org/pipermail/freebsd-net/2017-August/048621.html. TL;DR is this: https://lists.freebsd.org/pipermail/freebsd-net/2017-September/048826.html