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/sched/sch_sfq.c b/net/sched/sch_sfq.c
index c2e628d..7ef87f9 100644
--- a/net/sched/sch_sfq.c
+++ b/net/sched/sch_sfq.c
@@ -169,7 +169,7 @@
 	}
 	case htons(ETH_P_IPV6):
 	{
-		struct ipv6hdr *iph;
+		const struct ipv6hdr *iph;
 		int poff;
 
 		if (!pskb_network_may_pull(skb, sizeof(*iph)))