bridge: mcast: Account for missing attributes

The 'MDBA_MDB' and 'MDBA_MDB_ENTRY' nest attributes are not accounted
for when calculating the size of MDB notifications. Add them along with
comments for existing attributes.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Acked-by: Nikolay Aleksandrov <razor@blackwall.org>
Signed-off-by: David S. Miller <davem@davemloft.net>