net: mana: Use struct_size() helper in mana_gd_create_dma_region()

Make use of the struct_size() helper instead of an open-coded version,
in order to avoid any potential type mistakes or integer overflows that,
in the worst scenario, could lead to heap overflows.

Also, address the following sparse warnings:
drivers/net/ethernet/microsoft/mana/gdma_main.c:677:24: warning: using sizeof on a flexible structure

Link: https://github.com/KSPP/linux/issues/174Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Dexuan Cui <decui@microsoft.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/microsoft/mana/gdma_main.c

@@ -663,7 +663,7 @@ static int mana_gd_create_dma_region(struct gdma_dev *gd,
 	struct gdma_context *gc = gd->gdma_context;
 	struct hw_channel_context *hwc;
 	u32 length = gmi->length;
-	u32 req_msg_size;
+	size_t req_msg_size;
 	int err;
 	int i;
 
@@ -674,7 +674,7 @@ static int mana_gd_create_dma_region(struct gdma_dev *gd,
 		return -EINVAL;
 
 	hwc = gc->hwc.driver_data;
-	req_msg_size = sizeof(*req) + num_page * sizeof(u64);
+	req_msg_size = struct_size(req, page_addr_list, num_page);
 	if (req_msg_size > hwc->max_req_msg_size)
 		return -EINVAL;