udp: properly flush normal packet at GRO time

If an UDP packet enters the GRO engine but is not eligible
for aggregation and is not targeting an UDP tunnel,
udp_gro_receive() will not set the flush bit, and packet
could delayed till the next napi flush.

Fix the issue ensuring non GROed packets traverse
skb_gro_flush_final().
Reported-and-tested-by: Matthias Treydte <mt@waldheinz.de>
Fixes: 18f25dc3 ("udp: skip L4 aggregation for UDP tunnel packets")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/udp_offload.c

@@ -525,8 +525,10 @@ struct sk_buff *udp_gro_receive(struct list_head *head, struct sk_buff *skb,
 
 		if ((!sk && (skb->dev->features & NETIF_F_GRO_UDP_FWD)) ||
 		    (sk && udp_sk(sk)->gro_enabled) || NAPI_GRO_CB(skb)->is_flist)
-			pp = call_gro_receive(udp_gro_receive_segment, head, skb);
-		return pp;
+			return call_gro_receive(udp_gro_receive_segment, head, skb);
+
+		/* no GRO, be sure flush the current packet */
+		goto out;
 	}
 
 	if (NAPI_GRO_CB(skb)->encap_mark ||