net: Add MDB bulk deletion device operation

Add MDB net device operation that will be invoked by rtnetlink code in
response to received 'RTM_DELMDB' messages with the 'NLM_F_BULK' flag
set. Subsequent patches will implement the operation in the bridge and
VXLAN drivers.

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