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: Steven Noonan <steven@uplinklabs.net>
Reported-by: Randy Dunlap <randy.dunlap@oracle.com>
Reported-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>