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/key/af_key.c b/net/key/af_key.c
index 7db86ff..d62401c 100644
--- a/net/key/af_key.c
+++ b/net/key/af_key.c
@@ -712,7 +712,7 @@
 		sin6->sin6_family = AF_INET6;
 		sin6->sin6_port = port;
 		sin6->sin6_flowinfo = 0;
-		ipv6_addr_copy(&sin6->sin6_addr, (struct in6_addr *)xaddr->a6);
+		ipv6_addr_copy(&sin6->sin6_addr, (const struct in6_addr *)xaddr->a6);
 		sin6->sin6_scope_id = 0;
 		return 128;
 	    }