net: Fix dev_mc_add()

Commit 6e17d45a (net: add addr len check to dev_mc_add)
added a bug in dev_mc_add(), since it can now exit with a lock
imbalance.
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
CC: Jiri Pirko <jpirko@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/dev_mcast.c

@@ -97,7 +97,8 @@ int dev_mc_add(struct net_device *dev, void *addr, int alen, int glbl)
 
 	netif_addr_lock_bh(dev);
 	if (alen != dev->addr_len)
-		return -EINVAL;
+		err = -EINVAL;
+	else
 		err = __dev_addr_add(&dev->mc_list, &dev->mc_count, addr, alen, glbl);
 	if (!err)
 		__dev_set_rx_mode(dev);