landlock: Define access_mask_t to enforce a consistent access mask size

Create and use the access_mask_t typedef to enforce a consistent access
mask size and uniformly use a 16-bits type.  This will helps transition
to a 32-bits value one day.

Add a build check to make sure all (filesystem) access rights fit in.
This will be extended with a following commit.

Reviewed-by: Paul Moore <paul@paul-moore.com>
Link: https://lore.kernel.org/r/20220506161102.525323-2-mic@digikod.net


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