selftests/landlock: Replace access_fs_16 with ACCESS_ALL in fs_test

selftests/landlock: Replace access_fs_16 with ACCESS_ALL in fs_test

The access_fs_16 variable was originally intended to stay frozen at 16
access rights so that audit tests would not need updating when new
access rights are added.  Now that we have 17 access rights, the name
is confusing.

Replace all uses of access_fs_16 with ACCESS_ALL and delete the
variable.

Suggested-by: Mickaël Salaün <mic@digikod.net>
Signed-off-by: Günther Noack <gnoack3000@gmail.com>
Link: https://lore.kernel.org/r/20260327164838.38231-8-gnoack3000@gmail.com
Signed-off-by: Mickaël Salaün <mic@digikod.net>