devlink: don't work with possible NULL pointer in devlink_param_unregister()

There is a WARN_ON checking the param_item for being NULL when the param
is not inserted in the list. That indicates a driver BUG. Instead of
continuing to work with NULL pointer with its consequences, return.
Signed-off-by: NJiri Pirko <jiri@nvidia.com>
Reviewed-by: NJakub Kicinski <kuba@kernel.org>
Reviewed-by: NJacob Keller <jacob.e.keller@intel.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/devlink/leftover.c

@@ -10824,7 +10824,8 @@ static void devlink_param_unregister(struct devlink *devlink,
 
 	param_item =
 		devlink_param_find_by_name(&devlink->param_list, param->name);
-	WARN_ON(!param_item);
+	if (WARN_ON(!param_item))
+		return;
 	devlink_param_notify(devlink, 0, param_item, DEVLINK_CMD_PARAM_DEL);
 	list_del(&param_item->list);
 	kfree(param_item);