Stop sending tiny new data segments during SACK recovery

Consider the currently in-use TCP options when
calculating the amount of new data to be injected during
SACK loss recovery. That addresses the effect that very small
(new) segments could be injected on partial ACKs while
still performing a SACK loss recovery.

Reported by: Liang Tian
Reviewed by: tuexen, chengc_netapp.com
MFC after: 2 weeks
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D26446