net: bridge: Send notification when host join/leaves a group

The host can join or leave a multicast group on the brX interface, as
indicated by IGMP snooping.  This is tracked within the bridge
multicast code. Send a notification when this happens, in the same way
a notification is sent when a port of the bridge joins/leaves a group
because of IGMP snooping.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Acked-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>