drop_monitor: Add basic infrastructure for hardware drops

Export a function that can be invoked in order to report packets that
were dropped by the underlying hardware along with metadata.

Subsequent patches will add support for the different alert modes.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>