landlock: Create find_rule() from unmask_layers()

commit 2cd7cd6eed88b8383cfddce589afe9c0ae1d19b4 upstream.

This refactoring will be useful in a following commit.

Reviewed-by: Paul Moore <paul@paul-moore.com>
Link: https://lore.kernel.org/r/20220506161102.525323-4-mic@digikod.net


Cc: stable@vger.kernel.org
Signed-off-by: Mickaël Salaün <mic@digikod.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>