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/sctp/ipv6.c b/net/sctp/ipv6.c
index 865ce7b..321f175 100644
--- a/net/sctp/ipv6.c
+++ b/net/sctp/ipv6.c
@@ -531,7 +531,7 @@
 static int sctp_v6_available(union sctp_addr *addr, struct sctp_sock *sp)
 {
 	int type;
-	struct in6_addr *in6 = (struct in6_addr *)&addr->v6.sin6_addr;
+	const struct in6_addr *in6 = (const struct in6_addr *)&addr->v6.sin6_addr;
 
 	type = ipv6_addr_type(in6);
 	if (IPV6_ADDR_ANY == type)