mlx4: fix use-after-free in mlx4_en_fold_software_stats()

My recent commit to get more precise rx/tx counters in ndo_get_stats64()
can lead to crashes at device dismantle, as Jesper found out.

We must prevent mlx4_en_fold_software_stats() trying to access
tx/rx rings if they are deleted.

Fix this by adding a test against priv->port_up in
mlx4_en_fold_software_stats()

Calling mlx4_en_fold_software_stats() from mlx4_en_stop_port()
allows us to eventually broadcast the latest/current counters to
rtnetlink monitors.

Fixes: 40931b85 ("mlx4: give precise rx/tx bytes/packets counters")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-and-bisected-by: Jesper Dangaard Brouer <brouer@redhat.com>
Tested-by: Jesper Dangaard Brouer <brouer@redhat.com>
Cc: Tariq Toukan <tariqt@mellanox.com>
Cc: Saeed Mahameed <saeedm@dev.mellanox.co.il>
Acked-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

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

@@ -1809,8 +1809,12 @@ void mlx4_en_stop_port(struct net_device *dev, int detach)
 
 	netif_tx_disable(dev);
 
+	spin_lock_bh(&priv->stats_lock);
+	mlx4_en_fold_software_stats(dev);
 	/* Set port as not active */
 	priv->port_up = false;
+	spin_unlock_bh(&priv->stats_lock);
+
 	priv->counter_index = MLX4_SINK_COUNTER_INDEX(mdev->dev);
 
 	/* Promsicuous mode */

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

@@ -154,7 +154,7 @@ void mlx4_en_fold_software_stats(struct net_device *dev)
 	unsigned long packets, bytes;
 	int i;
 
-	if (mlx4_is_master(mdev->dev))
+	if (!priv->port_up || mlx4_is_master(mdev->dev))
 		return;
 
 	packets = 0;