Add MDB get attributes that correspond to the MDB set attributes used in
RTM_NEWMDB messages. Specifically, add 'MDBA_GET_ENTRY' which will hold
a 'struct br_mdb_entry' and 'MDBA_GET_ENTRY_ATTRS' which will hold
'MDBE_ATTR_*' attributes that are used as indexes (source IP and source
VNI).
Signed-off-by: Ido Schimmel <idosch@nvidia.com> Acked-by: Nikolay Aleksandrov <razor@blackwall.org> Signed-off-by: David S. Miller <davem@davemloft.net>