net/mlx5: fix statistics read on Linux

This patch encompasses a few fixes carried by a previous patch
that aimed to support bonding device stats counting.

- If mlx5_os_read_dev_stat fails, it returns 1 instead of a
  negative value, causing mlx5_xstats_get to return an invalid
  number of counters. Since this error is not blocking, do not
  mess ret value with mlx5_os_read_dev_stat returned value.

  This allows avoiding the very annoying log:
  "n_xstats != n_xstats_names => skipping"

- Invert the check for mlx5_os_read_dev_stat(), currently leading
  us to store the result if the function failed, and use a
  backup value if it succeeded, which is the opposite of what we
  actually want. Revert to the original (correct) test.

- Add missing test on _mlx5_os_read_dev_counters() to prevent
  using trash stats values.

Fixes: 7ed15acdcd69 ("net/mlx5: improve xstats of bonding port")
Cc: stable@dpdk.org

Signed-off-by: Didier Pallard <didier.pallard@6wind.com>
Signed-off-by: Geoffrey Le Gourriérec <geoffrey.le_gourrierec@6wind.com>
Tested-by: Bassam Zaid AlKilani <bzalkilani@nvidia.com>
Acked-by: Matan Azrad <matan@nvidia.com>

drivers/net/mlx5/linux/mlx5_ethdev_os.c

@@ -1386,15 +1386,16 @@ mlx5_os_read_dev_counters(struct rte_eth_dev *dev, uint64_t *stats)
 		}
 	} else {
 		ret = _mlx5_os_read_dev_counters(dev, -1, stats);
+		if (ret)
+			return ret;
 	}
 	/* Read IB counters. */
 	for (i = 0; i != xstats_ctrl->mlx5_stats_n; ++i) {
 		if (!xstats_ctrl->info[i].dev)
 			continue;
-		ret = mlx5_os_read_dev_stat(priv, xstats_ctrl->info[i].ctr_name,
-					    &stats[i]);
 		/* return last xstats counter if fail to read. */
-		if (ret != 0)
+		if (mlx5_os_read_dev_stat(priv, xstats_ctrl->info[i].ctr_name,
+			    &stats[i]) == 0)
 			xstats_ctrl->xstats[i] = stats[i];
 		else
 			stats[i] = xstats_ctrl->xstats[i];