IB/mlx4: Fix shutdown crash accessing a non-existent bitmap

Commit cfcde11c3d7a ("IB/mlx4: Use flow counters on IBoE ports") added
code that sets elements of counters[] to -1 if no counter is allocated,
but then goes ahead and passes every entry to mlx4_counter_free() on
shutdown.  This is a bad idea, especially if MLX4_DEV_CAP_FLAG_COUNTERS
isn't set so there isn't even an underlying bitmap to free from.

Tested-by: Sean Hefty <sean.hefty@intel.com>
Cc: <stable@kernel.org>
Signed-off-by: Roland Dreier <roland@purestorage.com>

drivers/infiniband/hw/mlx4/main.c

@@ -1244,7 +1244,8 @@ err_reg:
 
 err_counter:
 	for (; i; --i)
-		mlx4_counter_free(ibdev->dev, ibdev->counters[i - 1]);
+		if (ibdev->counters[i - 1] != -1)
+			mlx4_counter_free(ibdev->dev, ibdev->counters[i - 1]);
 
 err_map:
 	iounmap(ibdev->uar_map);
@@ -1275,7 +1276,8 @@ static void mlx4_ib_remove(struct mlx4_dev *dev, void *ibdev_ptr)
 	}
 	iounmap(ibdev->uar_map);
 	for (p = 0; p < ibdev->num_ports; ++p)
-		mlx4_counter_free(ibdev->dev, ibdev->counters[p]);
+		if (ibdev->counters[p] != -1)
+			mlx4_counter_free(ibdev->dev, ibdev->counters[p]);
 	mlx4_foreach_port(p, dev, MLX4_PORT_TYPE_IB)
 		mlx4_CLOSE_PORT(dev, p);