netfilter: xtables: fix IPv6 dependency in the cluster match

This patch fixes a dependency with IPv6:

ERROR: "__ipv6_addr_type" [net/netfilter/xt_cluster.ko] undefined!

This patch adds a function that checks if the higher bits of the
address is 0xFF to identify a multicast address, instead of adding a
dependency due to __ipv6_addr_type(). I came up with this idea after
Patrick McHardy pointed possible problems with runtime module
dependencies.
Reported-by: NSteven Noonan <steven@uplinklabs.net>
Reported-by: NRandy Dunlap <randy.dunlap@oracle.com>
Reported-by: NCyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: NPablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/netfilter/xt_cluster.c

@@ -57,6 +57,13 @@ xt_cluster_hash(const struct nf_conn *ct,
 	return (((u64)hash * info->total_nodes) >> 32);
 }
 
+static inline bool
+xt_cluster_ipv6_is_multicast(const struct in6_addr *addr)
+{
+	__be32 st = addr->s6_addr32[0];
+	return ((st & htonl(0xFF000000)) == htonl(0xFF000000));
+}
+
 static inline bool
 xt_cluster_is_multicast_addr(const struct sk_buff *skb, u_int8_t family)
 {
@@ -67,8 +74,8 @@ xt_cluster_is_multicast_addr(const struct sk_buff *skb, u_int8_t family)
 		is_multicast = ipv4_is_multicast(ip_hdr(skb)->daddr);
 		break;
 	case NFPROTO_IPV6:
-		is_multicast = ipv6_addr_type(&ipv6_hdr(skb)->daddr) &
-						IPV6_ADDR_MULTICAST;
+		is_multicast =
+			xt_cluster_ipv6_is_multicast(&ipv6_hdr(skb)->daddr);
 		break;
 	default:
 		WARN_ON(1);