MFC r239672 (by rrs):

MFC r239672 (by rrs):
  This small change takes care of a race condition
  that can occur when both sides close at the same time.
  If that occurs, without this fix the connection enters
  FIN1 on both sides and they will forever send FIN|ACK at
  each other until the connection times out. This is because
  we stopped processing the FIN|ACK and thus did not advance
  the sequence and so never ACK'd each others FIN. This
  fix adjusts it so we *do* process the FIN properly and
  the race goes away ;-)

Approved by:	rrs
Approved by:	emaste (co-mentor)

git-svn-id: svn://svn.freebsd.org/base/stable/9@246997 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f