tcp: fix detection of bad RTOs

If timestamps are enabled, the actions performed by a retransmission
timeout were rolled back, when they should not.
It is needed to make sure the incoming segment advances SND.UNA.
To do this, remove the incorrect upfront check and extend the check in
the fast path to handle also the case of timestamps.

PR: 282605
Reviewed by: cc, rscheff, Peter Lei
MFC after: 3 days
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D49414