ip_tunnel: do not clear l4 hashes

If skb has a valid l4 hash, there is no point clearing hash and force
a further flow dissection when a tunnel encapsulation is added.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/ip_tunnel_core.c

@@ -69,7 +69,7 @@ void iptunnel_xmit(struct sock *sk, struct rtable *rt, struct sk_buff *skb,
 
 	skb_scrub_packet(skb, xnet);
 
-	skb_clear_hash(skb);
+	skb_clear_hash_if_not_l4(skb);
 	skb_dst_set(skb, &rt->dst);
 	memset(IPCB(skb), 0, sizeof(*IPCB(skb)));