landlock: Move filesystem helpers and add a new one

Move the SB_NOUSER and IS_PRIVATE dentry check to a standalone
is_nouser_or_private() helper.  This will be useful for a following
commit.

Move get_mode_access() and maybe_remove() to make them usable by new
code provided by a following commit.

Reviewed-by: Paul Moore <paul@paul-moore.com>
Signed-off-by: Mickaël Salaün <mic@digikod.net>
Link: https://lore.kernel.org/r/20220506161102.525323-6-mic@digikod.net