gve: Replace zero-length array with flexible-array member

There is a regular need in the kernel to provide a way to declare having a
dynamically sized set of trailing elements in a structure.  Kernel code
should always use “flexible array members”[1] for these cases.  The
older style of one-element or zero-length arrays should no longer be
used[2].

Refactor the code according to the use of a flexible-array member in
struct gve_stats_report, instead of a zero-length array, and use the
struct_size() helper to calculate the size for the resource allocation.

[1] https://en.wikipedia.org/wiki/Flexible_array_member
[2] https://www.kernel.org/doc/html/v5.9/process/deprecated.html#zero-length-and-one-element-arraysSigned-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>

drivers/net/ethernet/google/gve/gve_adminq.h

@@ -198,7 +198,7 @@ static_assert(sizeof(struct stats) == 16);
 
 struct gve_stats_report {
 	__be64 written_count;
-	struct stats stats[0];
+	struct stats stats[];
 };
 
 static_assert(sizeof(struct gve_stats_report) == 8);

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

@@ -116,9 +116,8 @@ static int gve_alloc_stats_report(struct gve_priv *priv)
 		       priv->tx_cfg.num_queues;
 	rx_stats_num = (GVE_RX_STATS_REPORT_NUM + NIC_RX_STATS_REPORT_NUM) *
 		       priv->rx_cfg.num_queues;
-	priv->stats_report_len = sizeof(struct gve_stats_report) +
-				 (tx_stats_num + rx_stats_num) *
-				 sizeof(struct stats);
+	priv->stats_report_len = struct_size(priv->stats_report, stats,
+					     tx_stats_num + rx_stats_num);
 	priv->stats_report =
 		dma_alloc_coherent(&priv->pdev->dev, priv->stats_report_len,
 				   &priv->stats_report_bus, GFP_KERNEL);