landlock: Use bit-fields for storing handled layer access masks

When defined using bit-fields, the compiler takes care of packing the
bits in a memory-efficient way and frees us from defining
LANDLOCK_SHIFT_ACCESS_* by hand.  The exact memory layout does not
matter in our use case.

The manual definition of LANDLOCK_SHIFT_ACCESS_* has resulted in bugs in
at least two recent patch sets [1] [2] where new kinds of handled access
rights were introduced.

Cc: Mikhail Ivanov <ivanov.mikhail1@huawei-partners.com>
Cc: Tahera Fahimi <fahimitahera@gmail.com>
Link: https://lore.kernel.org/r/ebd680cc-25d6-ee14-4856-310f5e5e28e4@huawei-partners.com [1]
Link: https://lore.kernel.org/r/ZmLEoBfHyUR3nKAV@google.com

 [2]
Signed-off-by: Günther Noack <gnoack@google.com>
Link: https://lore.kernel.org/r/20240610082115.1693267-1-gnoack@google.com


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