gve: fix gve_get_stats()

gve_get_stats() can report wrong numbers if/when u64_stats_fetch_retry()
returns true.

What is needed here is to sample values in temporary variables,
and only use them after each loop is ended.

Fixes: f5cedc84 ("gve: Add transmit and receive support")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Catherine Sullivan <csully@google.com>
Cc: Sagi Shahar <sagis@google.com>
Cc: Jon Olson <jonolson@google.com>
Cc: Willem de Bruijn <willemb@google.com>
Cc: Luigi Rizzo <lrizzo@google.com>
Cc: Jeroen de Borst <jeroendb@google.com>
Cc: Tao Liu <xliutaox@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/google/gve/gve_main.c

@@ -41,6 +41,7 @@ static void gve_get_stats(struct net_device *dev, struct rtnl_link_stats64 *s)
 {
 	struct gve_priv *priv = netdev_priv(dev);
 	unsigned int start;
+	u64 packets, bytes;
 	int ring;
 
 	if (priv->rx) {
@@ -48,10 +49,12 @@ static void gve_get_stats(struct net_device *dev, struct rtnl_link_stats64 *s)
 			do {
 				start =
 				  u64_stats_fetch_begin(&priv->rx[ring].statss);
-				s->rx_packets += priv->rx[ring].rpackets;
-				s->rx_bytes += priv->rx[ring].rbytes;
+				packets = priv->rx[ring].rpackets;
+				bytes = priv->rx[ring].rbytes;
 			} while (u64_stats_fetch_retry(&priv->rx[ring].statss,
 						       start));
+			s->rx_packets += packets;
+			s->rx_bytes += bytes;
 		}
 	}
 	if (priv->tx) {
@@ -59,10 +62,12 @@ static void gve_get_stats(struct net_device *dev, struct rtnl_link_stats64 *s)
 			do {
 				start =
 				  u64_stats_fetch_begin(&priv->tx[ring].statss);
-				s->tx_packets += priv->tx[ring].pkt_done;
-				s->tx_bytes += priv->tx[ring].bytes_done;
+				packets = priv->tx[ring].pkt_done;
+				bytes = priv->tx[ring].bytes_done;
 			} while (u64_stats_fetch_retry(&priv->tx[ring].statss,
 						       start));
+			s->tx_packets += packets;
+			s->tx_bytes += bytes;
 		}
 	}
 }