ipvlan: fix incorrect usage of IS_ERR() macro in IPv6 code path.

The ip6_route_output() always returns a valid dst pointer unlike in IPv4
case. So the validation has to be different from the IPv4 path. Correcting
that error in this patch.

This was picked up by a static checker with a following warning -

   drivers/net/ipvlan/ipvlan_core.c:380 ipvlan_process_v6_outbound()
        warn: 'dst' isn't an ERR_PTR
Signed-off-by: Mahesh Bandewar <maheshb@google.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ipvlan/ipvlan_core.c

@@ -377,9 +377,11 @@ static int ipvlan_process_v6_outbound(struct sk_buff *skb)
 	};
 
 	dst = ip6_route_output(dev_net(dev), NULL, &fl6);
-	if (IS_ERR(dst))
+	if (dst->error) {
+		ret = dst->error;
+		dst_release(dst);
 		goto err;
-
+	}
 	skb_dst_drop(skb);
 	skb_dst_set(skb, dst);
 	err = ip6_local_out(skb);