net/mlx4_core: Fix wrong calculation of free counters

The field res_free indicates the total number of counters which are
available for allocation (reserved and unreserved). Fixed a bug where
the reserved counters were subtracted from res_free before any
allocation was performed.

Before this fix, free counters which were not reserved could not be
allocated.

Fixes: 9de92c60

 ("net/mlx4_core: Adjust counter grant policy in the resource tracker")
Signed-off-by: Eran Ben Elisha <eranbe@mellanox.com>
Reviewed-by: Jack Morgenstein <jackm@dev.mellanox.co.il>
Signed-off-by: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>