net: bridge: vlan: add support for mcast router global option

Add support to change and retrieve global vlan multicast router state
which is used for the bridge itself. We just need to pass multicast context
to br_multicast_set_router instead of bridge device and the rest of the
logic remains the same.

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