landlock: Document init_layer_masks() helper

Add kernel-doc to the init_layer_masks() function.

Signed-off-by: Günther Noack <gnoack3000@gmail.com>
Link: https://lore.kernel.org/r/20221018182216.301684-4-gnoack3000@gmail.com


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