net/mlx4_en: Support ndo_get_vf_stats

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: Eran Ben Elisha <eranbe@mellanox.com>
Signed-off-by: Hadar Hen Zion <hadarh@mellanox.com>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>