landlock: Reduce the maximum number of layers to 16

The maximum number of nested Landlock domains is currently 64.  Because
of the following fix and to help reduce the stack size, let's reduce it
to 16.  This seems large enough for a lot of use cases (e.g. sandboxed
init service, spawning a sandboxed SSH service, in nested sandboxed
containers).  Reducing the number of nested domains may also help to
discover misuse of Landlock (e.g. creating a domain per rule).

Add and use a dedicated layer_mask_t typedef to fit with the number of
layers.  This might be useful when changing it and to keep it consistent
with the maximum number of layers.

Reviewed-by: Paul Moore <paul@paul-moore.com>
Link: https://lore.kernel.org/r/20220506161102.525323-3-mic@digikod.net
Cc: stable@vger.kernel.org
Signed-off-by: Mickaël Salaün <mic@digikod.net>