tcp: Sanity check of SACK holes on retransmissions

Adding a few KASSERT() to validate sanity of sack holes, and
bail out if sack hole is inconsistent to avoid panicing non-invariant builds.

Reviewed By: hselasky, glebius
PR: 263445
MFC after: 1 week
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D35387