MFC 303766

MFC 303766
    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

git-svn-id: svn://svn.freebsd.org/base/stable/10@304836 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f