netfilter: xt_cluster: get rid of xt_cluster_ipv6_is_multicast

If use the ipv6_addr_is_multicast instead of xt_cluster_ipv6_is_multicast,
then we can reduce code size.

Signed-off-by: Taehee Yoo <ap420073@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>