net: Add MDB get device operation

Add MDB net device operation that will be invoked by rtnetlink code in
response to received RTM_GETMDB messages. Subsequent patches will
implement the operation in the bridge and VXLAN drivers.

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