Convert virtio_console to struct_size

Signed-off-by: Matthew Wilcox <mawilcox@microsoft.com>
Signed-off-by: Kees Cook <keescook@chromium.org>

drivers/char/virtio_console.c

@@ -433,8 +433,7 @@ static struct port_buffer *alloc_buf(struct virtio_device *vdev, size_t buf_size
 	 * Allocate buffer and the sg list. The sg list array is allocated
 	 * directly after the port_buffer struct.
 	 */
-	buf = kmalloc(sizeof(*buf) + sizeof(struct scatterlist) * pages,
-		      GFP_KERNEL);
+	buf = kmalloc(struct_size(buf, sg, pages), GFP_KERNEL);
 	if (!buf)
 		goto fail;