bridge: mcast: Use MDB configuration structure where possible

The MDB configuration structure (i.e., struct br_mdb_config) now
includes all the necessary information from the parsed RTM_{NEW,DEL}MDB
netlink messages, so use it.

This will later allow us to delete the calls to br_mdb_parse() from
br_mdb_add() and br_mdb_del().

No functional changes intended.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Acked-by: Nikolay Aleksandrov <razor@blackwall.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>