dccp: Evaluate ip_hdr() only once in dccp_v4_route_skb().

This also works around a bogus gcc warning generated by an
upcoming patch from Eric Dumazet that rearranges the layout
of struct flowi4.

Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/dccp/ipv4.c b/net/dccp/ipv4.c
index 3f4e541..1c67fe8 100644
--- a/net/dccp/ipv4.c
+++ b/net/dccp/ipv4.c
@@ -474,10 +474,11 @@
 					   struct sk_buff *skb)
 {
 	struct rtable *rt;
+	const struct iphdr *iph = ip_hdr(skb);
 	struct flowi4 fl4 = {
 		.flowi4_oif = skb_rtable(skb)->rt_iif,
-		.daddr = ip_hdr(skb)->saddr,
-		.saddr = ip_hdr(skb)->daddr,
+		.daddr = iph->saddr,
+		.saddr = iph->daddr,
 		.flowi4_tos = RT_CONN_FLAGS(sk),
 		.flowi4_proto = sk->sk_protocol,
 		.fl4_sport = dccp_hdr(skb)->dccph_dport,