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
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D35731

(cherry picked from commit 5b741298b11cdaec71877fa24b72764d1e697a33)