landlock: Move and rename layer helpers

Move and rename landlock_unmask_layers() and landlock_init_layer_masks()
helpers to ruleset.c to share them with Landlock network implementation
in following commits.

Signed-off-by: Konstantin Meskhidze <konstantin.meskhidze@huawei.com>
Link: https://lore.kernel.org/r/20231026014751.414649-6-konstantin.meskhidze@huawei.com


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