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/ipmr.c b/net/ipv4/ipmr.c
index 1f62eae..c81b9b6 100644
--- a/net/ipv4/ipmr.c
+++ b/net/ipv4/ipmr.c
@@ -1549,7 +1549,7 @@
 static void ip_encap(struct sk_buff *skb, __be32 saddr, __be32 daddr)
 {
 	struct iphdr *iph;
-	struct iphdr *old_iph = ip_hdr(skb);
+	const struct iphdr *old_iph = ip_hdr(skb);
 
 	skb_push(skb, sizeof(struct iphdr));
 	skb->transport_header = skb->network_header;