mlx4: do not use priv->stats_lock in mlx4_en_auto_moderation()

Per RX ring packets/bytes counters are not protected by global
priv->stats_lock.

Better not confuse the reader, and use READ_ONCE() to show we read
these counters without surrounding synchronization.

Interrupt moderation is best effort, and we do not really care of
ultra precise counters.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

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

@@ -1394,10 +1394,8 @@ static void mlx4_en_auto_moderation(struct mlx4_en_priv *priv)
 		return;
 
 	for (ring = 0; ring < priv->rx_ring_num; ring++) {
-		spin_lock_bh(&priv->stats_lock);
-		rx_packets = priv->rx_ring[ring]->packets;
-		rx_bytes = priv->rx_ring[ring]->bytes;
-		spin_unlock_bh(&priv->stats_lock);
+		rx_packets = READ_ONCE(priv->rx_ring[ring]->packets);
+		rx_bytes = READ_ONCE(priv->rx_ring[ring]->bytes);
 
 		rx_pkt_diff = ((unsigned long) (rx_packets -
 				priv->last_moder_packets[ring]));