tcp/lro: If timestamps mismatch or it's a FIN, force flush.

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