ipv6: Add icmp_echo_ignore_multicast support for ICMPv6

IPv4 has icmp_echo_ignore_broadcast to prevent responding to broadcast pings.
IPv6 needs a similar mechanism.

v1->v2:
- Remove NET_IPV6_ICMP_ECHO_IGNORE_MULTICAST.

Signed-off-by: Stephen Suryaputra <ssuryaextr@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>