tcp: don't use KTLS socket option on listening sockets

KTLS socket options make use of socket buffers, which are not
available for listening sockets.

Reported by: syzbot+a8829e888a93a4a04619@syzkaller.appspotmail.com
Reviewed by: jhb@
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D27948