Mfp: r296310,r296343

It looks like as with the safety belt of DELAY() fastened (*) we can
completely tear down and free all memory for TCP (after r281599).

(*) in theory a few ticks should be good enough to make sure the timers
are all really gone. Could we use a better matric here and check a
tcbcb count as an optimization?

PR: 164763
Reviewed by: gnn, emaste
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D5734