Implement the ndo to gather VF statistics through the PF.

All counters related to this VF are stored in a per slave
list, run over the slave's list and collect all statistics.
Signed-off-by: NEran Ben Elisha <eranbe@mellanox.com>
Signed-off-by: NHadar Hen Zion <hadarh@mellanox.com>
Signed-off-by: NOr Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>