netdevice: Add offload statistics ndo

Add a new ndo to return statistics for offloaded operation.
Since there can be many different offloaded operation with many
stats types, the ndo gets an attribute id by which it knows which
stats are wanted. The ndo also gets a void pointer to be cast according
to the attribute id.

Signed-off-by: Nogah Frankel <nogahf@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Reviewed-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>