[IPSEC]: Handle local_df in IPv4

When cleaning up the remaining users of dst_pmtu I noticed that
local_df wasn't being treated correctly in IPsec.  In fact, if
you socket's dst went over IPsec, local_df is essentailly ignored.

This patch fixes that.
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/xfrm4_output.c

@@ -78,7 +78,7 @@ static int xfrm4_tunnel_check_size(struct sk_buff *skb)
 
 	IPCB(skb)->flags |= IPSKB_XFRM_TUNNEL_SIZE;
 	
-	if (!(iph->frag_off & htons(IP_DF)))
+	if (!(iph->frag_off & htons(IP_DF)) || skb->local_df)
 		goto out;
 
 	dst = skb->dst;