This keeps the segments/ACK/FIN delivery order.

Before this patch, it was observed: if A sent FIN immediately after
an ACK, B would deliver FIN first to the TCP stack, then the ACK.
This out-of-order delivery causes one unnecessary ACK sent from B.

Reviewed by:	gallatin, hps
Obtained from:  rrs, gallatin
Sponsored by:	Netflix (rrs, gallatin), Microsoft (sephe)
Differential Revision:	https://reviews.freebsd.org/D7415