Fix for ipv4 tunnel devices:

- do not make path mtu discovery, when it is disabled :-)

net/ipv4/ip_gre.c

@@ -760,7 +760,10 @@ static int ipgre_tunnel_xmit(struct sk_buff *skb, struct net_device *dev)
 	}
 
 	df = tiph->frag_off;
-	mtu = rt->u.dst.pmtu - tunnel->hlen;
+	if (df)
+		mtu = rt->u.dst.pmtu - tunnel->hlen;
+	else
+		mtu = skb->dst ? skb->dst->pmtu : dev->mtu;
 
 	if (skb->protocol == __constant_htons(ETH_P_IP)) {
 		if (skb->dst && mtu < skb->dst->pmtu && mtu >= 68)

net/ipv4/ipip.c

@@ -572,7 +572,11 @@ static int ipip_tunnel_xmit(struct sk_buff *skb, struct net_device *dev)
 		goto tx_error;
 	}
 
-	mtu = rt->u.dst.pmtu - sizeof(struct iphdr);
+	if (tiph->frag_off)
+		mtu = rt->u.dst.pmtu - sizeof(struct iphdr);
+	else
+		mtu = skb->dst ? skb->dst->pmtu : dev->mtu;
+
 	if (mtu < 68) {
 		tunnel->stat.collisions++;
 		ip_rt_put(rt);

net/ipv6/sit.c

@@ -517,7 +517,11 @@ static int ipip6_tunnel_xmit(struct sk_buff *skb, struct net_device *dev)
 		goto tx_error;
 	}
 
-	mtu = rt->u.dst.pmtu - sizeof(struct iphdr);
+	if (tiph->frag_off)
+		mtu = rt->u.dst.pmtu - sizeof(struct iphdr);
+	else
+		mtu = skb->dst ? skb->dst->pmtu : dev->mtu;
+
 	if (mtu < 68) {
 		tunnel->stat.collisions++;
 		ip_rt_put(rt);