net: devlink: add RNLT lock assertion to devlink_compat_switch_id_get()

Similar to devlink_compat_phys_port_name_get(), make sure that
devlink_compat_switch_id_get() is called with RTNL lock held. Comment
already says so, so put this in code as well.
Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Link: https://lore.kernel.org/r/20220825112923.1359194-1-jiri@resnulli.usSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/core/devlink.c

@@ -12505,6 +12505,8 @@ int devlink_compat_switch_id_get(struct net_device *dev,
 	 * devlink_port instance cannot disappear in the middle. No need to take
 	 * any devlink lock as only permanent values are accessed.
 	 */
+	ASSERT_RTNL();
+
 	devlink_port = netdev_to_devlink_port(dev);
 	if (!devlink_port || !devlink_port->switch_port)
 		return -EOPNOTSUPP;