landlock: Move code to ease future backports

To ease backports in setup.c, let's group changes from
__lsm_ro_after_init to __ro_after_init with commit f22f9aaf
("selinux: remove the runtime disable functionality"), and the
landlock_lsmid addition with commit f3b8788c ("LSM: Identify modules
by more than name").

That will help to backport the following errata.

Cc: Günther Noack <gnoack@google.com>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20250318161443.279194-2-mic@digikod.net


Fixes: f3b8788c ("LSM: Identify modules by more than name")
Signed-off-by: Mickaël Salaün <mic@digikod.net>