docs: update self-protection __ro_after_init status

__ro_after_init is no longer new and under development. Update the
document to reflect the correct status.

Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>
Link: https://lore.kernel.org/r/20211208164051.8783-1-skhan@linuxfoundation.org


Signed-off-by: Jonathan Corbet <corbet@lwn.net>