net: bridge: mdb: use mdb and port entries in notifications

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: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>