xenbus: fix overflow check in xenbus_dev_write()

Reported-by: Dan Carpenter <dan.carpenter@xxxxxxxxxx>
Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/drivers/xen/xenbus/xenbus_dev.c
+++ b/drivers/xen/xenbus/xenbus_dev.c
@@ -239,7 +239,7 @@ static ssize_t xenbus_dev_write(struct f
 	if (!is_xenstored_ready())
 		return -ENODEV;
 
-	if ((len + u->len) > sizeof(u->u.buffer)) {
+	if (len > sizeof(u->u.buffer) - u->len) {
 		rc = -EINVAL;
 		goto out;
 	}