net: devlink: add WARN_ON_ONCE to check return value of unregister_netdevice_notifier_net() call

As the return value is not 0 only in case there is no such notifier
block registered, add a WARN_ON_ONCE() to yell about it.
Suggested-by: Ido Schimmel <idosch@idosch.org>
Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Link: https://lore.kernel.org/r/20221125100255.1786741-1-jiri@resnulli.usSigned-off-by: Paolo Abeni <pabeni@redhat.com>

net/core/devlink.c

@@ -9907,8 +9907,8 @@ void devlink_free(struct devlink *devlink)
 
 	xa_destroy(&devlink->snapshot_ids);
 
-	unregister_netdevice_notifier_net(devlink_net(devlink),
-					  &devlink->netdevice_nb);
+	WARN_ON_ONCE(unregister_netdevice_notifier_net(devlink_net(devlink),
+						       &devlink->netdevice_nb));
 
 	xa_erase(&devlinks, devlink->index);