netfilter: pkttype: unnecessary to check ipv6 multicast address

Since there's no broadcast address in IPV6, so in ipv6 family, the
PACKET_LOOPBACK must be multicast packets, there's no need to check
it again.

Signed-off-by: Liping Zhang <zlpnobody@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>