landlock: Change landlock_add_rule(2) argument check ordering

This makes more sense to first check the ruleset FD and then the rule
attribute.  It will be useful to factor out code for other rule types.

Add inval_add_rule_arguments tests, extension of empty_path_beneath_attr
tests, to also check error ordering for landlock_add_rule(2).

Link: https://lore.kernel.org/r/20220506160820.524344-9-mic@digikod.net


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