tcp: stop spurious rescue retransmissions and potential asserts

Reported by: pho@
MFC after: 3 days
Reviewed By: tuexen, transport
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D29970

(cherry picked from commit 48be5b976e34022862da3be69350d60e969d39e5)