Send consistent SEG.WIN when using timewait codepath for TCP.

When sending TCP segments from the timewait code path, a stored
value of the last sent window is used. Use the same code for
computing this in the timewait code path as in the main code
path used in tcp_output() to avoiv inconsistencies.

Reviewed by: rrs@
MFC after: 1 month
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D16503