Check the V_tcbinfo.ipi_count to hit 0 before doing the full TCP cleanup.
That way timers can finish cleanly and we do not gamble with a DELAY().

Reviewed by: gnn, jtl
Approved by: re (gjb)
Obtained from: projects/vnet
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D6923