Restrict enabling TCP-FASTOPEN to end-points in CLOSED or LISTEN state

Enabling TCP-FASTOPEN on an end-point which is in a state other than
CLOSED or LISTEN, is a bug in the application. So it should not work.
Also the TCP code does not (and needs not to) handle this.
While there, also simplify the setting of the TF_FASTOPEN flag.

This issue was found by running syzkaller.

Reviewed by: rrs
MFC after: 1 week
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D25115