tcp: Undo the consumption of sequence space by FIN in case of a transient error.

If an error occurs while processing a TCP segment with some data and the FIN
flag, the back out of the sequence number advance does not take into account the
increase by 1 due to the FIN flag.

Reviewed By: jch, gnn, transport, tuexen
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D2970

(cherry picked from commit 66605ff791b12a2c3bb4570379db0e14d29fca4c)