bridge: mcast: Remove br_mdb_parse()

The parsing of the netlink messages and the validity checks are now
performed in br_mdb_config_init() so we can remove br_mdb_parse().

This finally allows us to stop passing netlink attributes deep in the
MDB control path and only use the MDB configuration structure.

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