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/bridge/br_netfilter.c b/net/bridge/br_netfilter.c
index f3bc322..5614907 100644
--- a/net/bridge/br_netfilter.c
+++ b/net/bridge/br_netfilter.c
@@ -219,7 +219,7 @@
 static int br_parse_ip_options(struct sk_buff *skb)
 {
 	struct ip_options *opt;
-	struct iphdr *iph;
+	const struct iphdr *iph;
 	struct net_device *dev = skb->dev;
 	u32 len;
 
@@ -554,7 +554,7 @@
 					   const struct net_device *out,
 					   int (*okfn)(struct sk_buff *))
 {
-	struct ipv6hdr *hdr;
+	const struct ipv6hdr *hdr;
 	u32 pkt_len;
 
 	if (skb->len < sizeof(struct ipv6hdr))