gve: Use size_add() in call to struct_size()

If, for any reason, `tx_stats_num + rx_stats_num` wraps around, the
protection that struct_size() adds against potential integer overflows
is defeated. Fix this by hardening call to struct_size() with size_add().

Fixes: 691f407 ("gve: Replace zero-length array with flexible-array member")
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

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

@@ -190,7 +190,7 @@ static int gve_alloc_stats_report(struct gve_priv *priv)
 	rx_stats_num = (GVE_RX_STATS_REPORT_NUM + NIC_RX_STATS_REPORT_NUM) *
 		       priv->rx_cfg.num_queues;
 	priv->stats_report_len = struct_size(priv->stats_report, stats,
-					     tx_stats_num + rx_stats_num);
+					     size_add(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);