bridge: mcast: Use MDB group key from configuration structure

The MDB group key (i.e., {source, destination, protocol, VID}) is
currently determined under the multicast lock from the netlink
attributes. Instead, use the group key from the MDB configuration
structure that was prepared before acquiring the lock.

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>