tcp: finish SACK loss recovery on sudden lack of SACK blocks

While a receiver should continue sending SACK blocks for the
duration of a SACK loss recovery, if for some reason the
TCP options no longer contain these SACK blocks, but we
already started maintaining the Scoreboard, keep on handling
incoming ACKs (without SACK) as belonging to the SACK recovery.

Reported by: thj
Reviewed by: tuexen, transport
MFC after: 2 weeks
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D36046