net/mlx4_core: Reset counters data when freed

Add resetting the counter data to the free counter flow, so the counter's
data won't be accessible anymore if querying the counter. Also, on next
counter allocation (to another VM for example), it will be fresh and clear.

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>