net: bridge: Rename mglist to host_joined

The boolean mglist indicates the host has joined a particular
multicast group on the bridge interface. It is badly named, obscuring
what is means. Rename it.

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>