net/mlx4_en: Disable RFS when running in SRIOV mode

Commit 37706996 "mlx4_en: fix allocation of CPU affinity reverse-map" fixed
a bug when mlx4_dev->caps.comp_pool is larger from the device rx rings, but
introduced a regression.

When the mlx4_core is activating its "legacy mode" (e.g when running in SRIOV
mode) w.r.t to EQs/IRQs usage, comp_pool becomes zero and we're crashing on
divide by zero alloc_cpu_rmap.

Fix that by enabling RFS only when running in non-legacy mode.
Reported-by: Yan Burman <yanb@mellanox.com>
Cc: Kleber Sacilotto de Souza <klebers@linux.vnet.ibm.com>
Signed-off-by: Amir Vadai <amirv@mellanox.com>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/mellanox/mlx4/en_netdev.c

@@ -1833,9 +1833,11 @@ int mlx4_en_alloc_resources(struct mlx4_en_priv *priv)
 	}
 
 #ifdef CONFIG_RFS_ACCEL
-	priv->dev->rx_cpu_rmap = alloc_irq_cpu_rmap(priv->mdev->dev->caps.comp_pool);
-	if (!priv->dev->rx_cpu_rmap)
-		goto err;
+	if (priv->mdev->dev->caps.comp_pool) {
+		priv->dev->rx_cpu_rmap = alloc_irq_cpu_rmap(priv->mdev->dev->caps.comp_pool);
+		if (!priv->dev->rx_cpu_rmap)
+			goto err;
+	}
 #endif
 
 	return 0;