6lowpan: double unlock on an error path

We already unlocked a few lines earlier here, so we can go directly to
drop without passing through unlock.  This was introduced recently in
c5d3687f ('6lowpan: read data from skb safely').
Signed-off-by: NDan Carpenter <dan.carpenter@oracle.com>
Acked-by: NAlexander Smirnov <alex.bluesman.smirnov@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ieee802154/6lowpan.c

@@ -771,7 +771,7 @@ lowpan_process_data(struct sk_buff *skb)
 			kfree(frame);
 
 			if (lowpan_fetch_skb_u8(skb, &iphc0))
-				goto unlock_and_drop;
+				goto drop;
 
 			break;
 		}