bridge: Export multicast enabled state

During enslavement to a bridge, after the CHANGEUPPER is sent, the
multicast enabled state of the bridge isn't propagated down to the
offloading driver unless it's changed.

This patch allows such drivers to query the multicast enabled state from
the bridge, so that they'll be able to correctly configure their flood
tables during port enslavement.

In case multicast is disabled, unregistered multicast packets can be
treated as broadcast and be flooded through all the bridge ports.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Reviewed-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>