bridge: mcast: add MLDv2 querier support

This patch adds basic support for MLDv2 queries, the default is MLDv1
as before. A new multicast option - multicast_mld_version, adds the
ability to change it between 1 and 2 via netlink and sysfs.
The MLD option is disabled if CONFIG_IPV6 is disabled.

Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/uapi/linux/if_link.h

@@ -276,6 +276,7 @@ enum {
 	IFLA_BR_VLAN_STATS_ENABLED,
 	IFLA_BR_MCAST_STATS_ENABLED,
 	IFLA_BR_MCAST_IGMP_VERSION,
+	IFLA_BR_MCAST_MLD_VERSION,
 	__IFLA_BR_MAX,
 };