[NETFILTER]: xt_mac/xt_CLASSIFY: use IPv6 hook names for IPv6 registration

Use NF_IP6_ instead of NF_IP_. The values are identical, this is merely
cleanup.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>