TCP Fast Open is triggered by sys_sendmsg with MSG_FASTOPEN flag for
SOCK_STREAM socket.

Even though it's sys_sendmsg, it eventually calls __inet_stream_connect
the same way sys_connect does for TCP.  __inet_stream_connect, in turn,
already has BPF hooks for sys_connect.

That means TFO is already covered by BPF_CGROUP_INET{4,6}_CONNECT and
the only missing piece is selftest. The patch adds selftest for TFO.
Signed-off-by: Andrey Ignatov <rdna@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>