tcp: no data on SYN segments unless doing TFO

Ensure that there is no data on SYN segments unless doing TFO.
This check is already in RACK and BBR.

Reported by: glebius
Reviewed by: rscheff
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D44384

(cherry picked from commit af700f430fd86ba3eae63e587985a12436db8f69)