system: introduce a new MlockState enum

system: introduce a new MlockState enum

Replace the boolean value enable_mlock with an enum and add a helper to
decide whether we should be calling os_mlock.

This is a stepping stone towards introducing a new mlock mode, which
will be the third possible state of this enum.

Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Daniil Tatianin <d-tatianin@yandex-team.ru>
Link: https://lore.kernel.org/r/20250212143920.1269754-4-d-tatianin@yandex-team.ru
Signed-off-by: Peter Xu <peterx@redhat.com>