bridge: mdb: Separate br_mdb_entry->state from net_bridge_port_group->state

Change net_bridge_port_group 'state' member to 'flags' and define new set
of flags internal to the kernel.

Signed-off-by: Elad Raz <eladr@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>