Check that in_pcbfree() is only called once for each PCB. If that

Check that in_pcbfree() is only called once for each PCB.  If that
assumption is violated, "bad things" could follow.

I believe such an assert would have detected some of the problems jch@
was chasing in PR 203175 (see r307551).  We also use it in our internal
TCP development efforts.  And, in case a bug does slip through to
released code, this change silently ignores subsequent calls to
in_pcbfree().

Reviewed by:	rrs
Sponsored by:	Netflix, Inc.
Differential Revision:	https://reviews.freebsd.org/D14990