[IPV6] MCAST: Ensure to check multicast listener(s).

In ip6_mc_input(), we need to check whether we have listener(s) for
the packet.

After commit ae7bf20a, all packets
for multicast destinations are delivered to upper layer if
IFF_PROMISC or IFF_ALLMULTI is set.

In fact, bug was rather ancient; the original (before the commit)
intent of the dev->flags check was to skip the ipv6_chk_mcast_addr()
call, assuming L2 filters packets appropriately, but it was even not
true.

Let's explicitly check our multicast list.

Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Acked-by: David L Stevens <dlstevens@us.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>