landlock: Optimize the number of calls to get_access_mask slightly

This call is now going through a function pointer,
and it is not as obvious any more that it will be inlined.

Signed-off-by: Günther Noack <gnoack@google.com>
Link: https://lore.kernel.org/r/20231208155121.1943775-4-gnoack@google.com
Fixes: 7a11275c

 ("landlock: Refactor layer helpers")
Signed-off-by: Mickaël Salaün <mic@digikod.net>