ipv6: disable GSO on sockets hitting dst_allfrag

If the allfrag feature has been set on a host route (due to an ICMPv6
Packet Too Big received indicating a MTU of less than 1280), we hit a
very slow behavior in TCP stack, because all big packets are dropped and
only a retransmit timer is able to push one MSS frame every 200 ms.

One way to handle this is to disable GSO on the socket the first time a
super packet is dropped. Adding a specific dst_allfrag() in the fast
path is probably overkill since the dst_allfrag() case almost never
happen.

Result on netperf TCP_STREAM, one flow :

Before : 60 kbit/sec
After : 1.6 Gbit/sec
Reported-by: Tore Anderson <tore@fud.no>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Tested-by: Tore Anderson <tore@fud.no>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/ip6_output.c

@@ -643,7 +643,10 @@ int ip6_fragment(struct sk_buff *skb, int (*output)(struct sk_buff *))
 	/* We must not fragment if the socket is set to force MTU discovery
 	 * or if the skb it not generated by a local socket.
 	 */
-	if (!skb->local_df && skb->len > mtu) {
+	if (unlikely(!skb->local_df && skb->len > mtu)) {
+		if (skb->sk && dst_allfrag(skb_dst(skb)))
+			sk_nocaps_add(skb->sk, NETIF_F_GSO_MASK);
+
 		skb->dev = skb_dst(skb)->dev;
 		icmpv6_send(skb, ICMPV6_PKT_TOOBIG, 0, mtu);
 		IP6_INC_STATS(net, ip6_dst_idev(skb_dst(skb)),