net: bridge: Notify on bridge device mrouter state changes

Add the SWITCHDEV_ATTR_ID_BRIDGE_MROUTER switchdev notification type, used
to indicate whether the bridge is or isn't mrouter. Notify when the bridge
changes its state, similarly to the already existing bridged port mrouter
notifications.

The notification uses the switchdev_attr.u.mrouter boolean flag to indicate
the current bridge mrouter status. Thus, it only indicates whether the
bridge is currently used as an mrouter or not, and does not indicate the
exact mrouter state of the bridge (learning, permanent, etc.).

Signed-off-by: Yotam Gigi <yotamg@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>