net: dsa: add support for BRIDGE_MROUTER attribute

This patch adds support for enabling or disabling the flooding of
unknown multicast traffic on the CPU ports, depending on the value
of the switchdev SWITCHDEV_ATTR_ID_BRIDGE_MROUTER attribute.

The current behavior is kept unchanged but a user can now prevent
the CPU conduit to be flooded with a lot of unregistered traffic that
the network stack needs to filter in software with e.g.:

    echo 0 > /sys/class/net/br0/multicast_router

Signed-off-by: Vivien Didelot <vivien.didelot@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>