Revert "net: ipv4: ip_forward: fix inverted local_df test"

This reverts commit 59d9f389, which
was commit ca6c5d4a upstream.  It is a
valid fix, but depends on sk_buff::local_df being set in all the right
cases, which it wasn't in 3.2.  We need to defer it unless and until
the other fixes are also backported to 3.2.y.
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

net/ipv4/ip_forward.c

@@ -42,12 +42,12 @@
 static bool ip_may_fragment(const struct sk_buff *skb)
 {
 	return unlikely((ip_hdr(skb)->frag_off & htons(IP_DF)) == 0) ||
-		skb->local_df;
+	       !skb->local_df;
 }
 
 static bool ip_exceeds_mtu(const struct sk_buff *skb, unsigned int mtu)
 {
-	if (skb->len <= mtu)
+	if (skb->len <= mtu || skb->local_df)
 		return false;
 
 	if (skb_is_gso(skb) && skb_gso_network_seglen(skb) <= mtu)