Prevent premature SACK block transmission during loss recovery

Under specific conditions, a window update can be sent with
outdated SACK information. Some clients react to this by
subsequently delaying loss recovery, making TCP perform very
poorly.

Reported by: chengc_netapp.com
Reviewed by: rrs, jtl
MFC after: 2 weeks
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D24237