devlink: introduce __devl_is_registered() helper and use it instead of xa_get_mark()

Introduce __devl_is_registered() which does not assert on devlink
instance lock and use it in notifications which may be called
without devlink instance lock held.

Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>