bridge: netlink: add support for multicast_startup_query_count

Add IFLA_BR_MCAST_STARTUP_QUERY_CNT to allow setting/getting
br->multicast_startup_query_count via netlink. Also align the ifla
comments.

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