sit: Clean up DF code by copying from IPIP

This patch rearranges the SIT DF bit handling using the new IPIP DF
code.  The only externally visible effect should be the case where
PMTU is enabled and the MTU is exactly 1280 bytes.  In this case the
previous code would send packets out with DF off while the new code
would set the DF bit.  This is inline with RFC 4213.
Signed-off-by: NHerbert Xu <herbert@gondor.apana.org.au>

Thanks,
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv6/sit.c

@@ -637,6 +637,7 @@ static netdev_tx_t ipip6_tunnel_xmit(struct sk_buff *skb,
 	struct iphdr  *tiph = &tunnel->parms.iph;
 	struct ipv6hdr *iph6 = ipv6_hdr(skb);
 	u8     tos = tunnel->parms.iph.tos;
+	__be16 df = tiph->frag_off;
 	struct rtable *rt;     			/* Route to the other host */
 	struct net_device *tdev;			/* Device to other host */
 	struct iphdr  *iph;			/* Our new IP header */
@@ -726,18 +727,20 @@ static netdev_tx_t ipip6_tunnel_xmit(struct sk_buff *skb,
 		goto tx_error;
 	}
 
-	if (tiph->frag_off)
+	if (df) {
 		mtu = dst_mtu(&rt->u.dst) - sizeof(struct iphdr);
-	else
-		mtu = skb_dst(skb) ? dst_mtu(skb_dst(skb)) : dev->mtu;
 
 		if (mtu < 68) {
 			stats->collisions++;
 			ip_rt_put(rt);
 			goto tx_error;
 		}
-	if (mtu < IPV6_MIN_MTU)
+
+		if (mtu < IPV6_MIN_MTU) {
 			mtu = IPV6_MIN_MTU;
+			df = 0;
+		}
+
 		if (tunnel->parms.iph.daddr && skb_dst(skb))
 			skb_dst(skb)->ops->update_pmtu(skb_dst(skb), mtu);
 
@@ -746,6 +749,7 @@ static netdev_tx_t ipip6_tunnel_xmit(struct sk_buff *skb,
 			ip_rt_put(rt);
 			goto tx_error;
 		}
+	}
 
 	if (tunnel->err_count > 0) {
 		if (time_before(jiffies,
@@ -792,11 +796,7 @@ static netdev_tx_t ipip6_tunnel_xmit(struct sk_buff *skb,
 	iph 			=	ip_hdr(skb);
 	iph->version		=	4;
 	iph->ihl		=	sizeof(struct iphdr)>>2;
-	if (mtu > IPV6_MIN_MTU)
-		iph->frag_off	=	tiph->frag_off;
-	else
-		iph->frag_off	=	0;
-
+	iph->frag_off		=	df;
 	iph->protocol		=	IPPROTO_IPV6;
 	iph->tos		=	INET_ECN_encapsulate(tos, ipv6_get_dsfield(iph6));
 	iph->daddr		=	rt->rt_dst;