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/ah4.c b/net/ipv4/ah4.c
index 4286fd3..c1f4154 100644
--- a/net/ipv4/ah4.c
+++ b/net/ipv4/ah4.c
@@ -73,7 +73,7 @@
  * into IP header for icv calculation. Options are already checked
  * for validity, so paranoia is not required. */
 
-static int ip_clear_mutable_options(struct iphdr *iph, __be32 *daddr)
+static int ip_clear_mutable_options(const struct iphdr *iph, __be32 *daddr)
 {
 	unsigned char * optptr = (unsigned char*)(iph+1);
 	int  l = iph->ihl*4 - sizeof(struct iphdr);
@@ -396,7 +396,7 @@
 static void ah4_err(struct sk_buff *skb, u32 info)
 {
 	struct net *net = dev_net(skb->dev);
-	struct iphdr *iph = (struct iphdr *)skb->data;
+	const struct iphdr *iph = (const struct iphdr *)skb->data;
 	struct ip_auth_hdr *ah = (struct ip_auth_hdr *)(skb->data+(iph->ihl<<2));
 	struct xfrm_state *x;
 
@@ -404,7 +404,8 @@
 	    icmp_hdr(skb)->code != ICMP_FRAG_NEEDED)
 		return;
 
-	x = xfrm_state_lookup(net, skb->mark, (xfrm_address_t *)&iph->daddr, ah->spi, IPPROTO_AH, AF_INET);
+	x = xfrm_state_lookup(net, skb->mark, (const xfrm_address_t *)&iph->daddr,
+			      ah->spi, IPPROTO_AH, AF_INET);
 	if (!x)
 		return;
 	printk(KERN_DEBUG "pmtu discovery on SA AH/%08x/%08x\n",