selftests/landlock: Test TCP accesses with protocol=IPPROTO_TCP

Extend protocol_variant structure with protocol field (Cf. socket(2)).

Extend protocol fixture with TCP test suits with protocol=IPPROTO_TCP
which can be used as an alias for IPPROTO_IP (=0) in socket(2).

Signed-off-by: Mikhail Ivanov <ivanov.mikhail1@huawei-partners.com>
Link: https://lore.kernel.org/r/20250205093651.1424339-3-ivanov.mikhail1@huawei-partners.com


Cc: <stable@vger.kernel.org> # 6.7.x
Signed-off-by: Mickaël Salaün <mic@digikod.net>