net: Add MDB net device operations

Add MDB net device operations that will be invoked by rtnetlink code in
response to received RTM_{NEW,DEL,GET}MDB messages. Subsequent patches
will implement these operations in the bridge and VXLAN drivers.

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