Fix the handling of TCP FIN-segments in the CLOSED state

When a TCP segment with the FIN bit set was received in the CLOSED state,
a TCP RST-ACK-segment is sent. When computing SEG.ACK for this, the
FIN counts as one byte. This accounting was missing and is fixed by this
patch.

Reviewed by: hiren
MFC after: 1 month
Sponsored by: Netflix, Inc.
Differential Revision: https://svn.freebsd.org/base/head