selftests/landlock: Add test for TCP fast open

selftests/landlock: Add test for TCP fast open

Enforce that TCP Fast Open is controlled by
LANDLOCK_ACCESS_NET_CONNECT_TCP. Semantics of connect() and
sendmsg(MSG_FASTOPEN) should be identical from Landlock's perspective.
Also enforce error code consistency, since UDP sockets ignore the
MSG_FASTOPEN flag while Unix sockets reject it.

Signed-off-by: Matthieu Buffet <matthieu@buffet.re>
Link: https://patch.msgid.link/20260701214628.33319-2-matthieu@buffet.re
Cc: stable@vger.kernel.org
[mic: Fix formatting]
Signed-off-by: Mickaël Salaün <mic@digikod.net>