lec: Use rtnl lock/unlock when updating MTU

The LECS response contains the MTU that should be used.  Correctly
synchronize with other layers when updating.
Signed-off-by: NChas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/atm/lec.c

@@ -410,9 +410,11 @@ static int lec_atm_send(struct atm_vcc *vcc, struct sk_buff *skb)
 		priv->lane2_ops = NULL;
 		if (priv->lane_version > 1)
 			priv->lane2_ops = &lane2_ops;
+		rtnl_lock();
 		if (dev_set_mtu(dev, mesg->content.config.mtu))
 			pr_info("%s: change_mtu to %d failed\n",
 				dev->name, mesg->content.config.mtu);
+		rtnl_unlock();
 		priv->is_proxy = mesg->content.config.is_proxy;
 		break;
 	case l_flush_tran_id: