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/core/netpoll.c b/net/core/netpoll.c
index 06be243..46d9c3a 100644
--- a/net/core/netpoll.c
+++ b/net/core/netpoll.c
@@ -539,7 +539,7 @@
 {
 	int proto, len, ulen;
 	int hits = 0;
-	struct iphdr *iph;
+	const struct iphdr *iph;
 	struct udphdr *uh;
 	struct netpoll_info *npinfo = skb->dev->npinfo;
 	struct netpoll *np, *tmp;