landlock: Factor out check_access_path()

Merge check_access_path() into current_check_access_path() and make
hook_path_mknod() use it.

Cc: Günther Noack <gnoack@google.com>
Link: https://lore.kernel.org/r/20250108154338.1129069-4-mic@digikod.net


Signed-off-by: Mickaël Salaün <mic@digikod.net>