The TFO server-side code contains some changes that are not conditioned on
the TCP_RFC7413 kernel option. This change removes those few instructions
from the packet processing path.

While not strictly necessary, for the sake of consistency, I applied the
new IS_FASTOPEN macro to all places in the packet processing path that
used the (t_flags & TF_FASTOPEN) check.

Reviewed by: hiren
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D8219