inet: constify ip headers and in6_addr

Add const qualifiers to structs iphdr, ipv6hdr and in6_addr pointers
where possible, to make code intention more obvious.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ipv4/xfrm4_state.c b/net/ipv4/xfrm4_state.c
index 1717c64..ea983ae 100644
--- a/net/ipv4/xfrm4_state.c
+++ b/net/ipv4/xfrm4_state.c
@@ -55,7 +55,7 @@
 
 int xfrm4_extract_header(struct sk_buff *skb)
 {
-	struct iphdr *iph = ip_hdr(skb);
+	const struct iphdr *iph = ip_hdr(skb);
 
 	XFRM_MODE_SKB_CB(skb)->ihl = sizeof(*iph);
 	XFRM_MODE_SKB_CB(skb)->id = iph->id;