tcp rack: fix switching to RACK when FIN has been sent
Fix the rack sendmap entry in case a FIN has been sent when the
stack is switched over to RACK.
Reported by: syzbot+dd55e316428419e9354b@syzkaller.appspotmail.com
Reviewed by: rrs
MFC after: 1 week
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D35731