tcp: ensure SACK rxmit never ends up left of its hole

When a RTO happens during SACK loss recovery, snd_recover can possibly pulled left.
With Lost Retransmission Detection (LRD) this can lead to rxmit of a hole to end up
pointing to the left of the hole, which is unexpected and leads to complications.

Reviewed By: tuexen, transport
Sponsored by: NetApp, Inc.
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D51725

(cherry picked from commit 65d4a83399843cb4c6dd44061599390843c162db)