We have to use mdb and port entries when sending mdb notifications in
order to fill in all group attributes properly. Before this change we
would've used a fake br_mdb_entry struct to fill in only partial
information about the mdb. Now we can also reuse the mdb dump fill
function and thus have only a single central place which fills the mdb
attributes.

v3: add IPv6 support
Signed-off-by: NNikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: NJakub Kicinski <kuba@kernel.org>