vrf: Fix use after free and double free in vrf_finish_output

Miguel reported an skb use after free / double free in vrf_finish_output
when neigh_output returns an error. The vrf driver should return after
the call to neigh_output as it takes over the skb on error path as well.

Patch is a simplified version of Miguel's patch which was written for 4.9,
and updated to top of tree.

Fixes: 8f58336 ("net: Add ethernet header for pass through VRF device")
Signed-off-by: Miguel Fadon Perlines <mfadon@teldat.com>
Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/vrf.c

@@ -578,12 +578,13 @@ static int vrf_finish_output(struct net *net, struct sock *sk, struct sk_buff *s
 	if (!IS_ERR(neigh)) {
 		sock_confirm_neigh(skb, neigh);
 		ret = neigh_output(neigh, skb);
+		rcu_read_unlock_bh();
+		return ret;
 	}
 
 	rcu_read_unlock_bh();
 err:
-	if (unlikely(ret < 0))
-		vrf_tx_error(skb->dev, skb);
+	vrf_tx_error(skb->dev, skb);
 	return ret;
 }