bridge: netlink: add support for multicast_last_member_count

Add IFLA_BR_MCAST_LAST_MEMBER_CNT to allow setting/getting
br->multicast_last_member_count via netlink.

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