xen/xenbus: fix overflow check in xenbus_file_write()

Acked-by: Ian Campbell <ian.campbell@citrix.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
[v1: Rebased on upstream]
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

drivers/xen/xenbus/xenbus_dev_frontend.c

@@ -458,7 +458,7 @@ static ssize_t xenbus_file_write(struct file *filp,
 		goto out;
 
 	/* Can't write a xenbus message larger we can buffer */
-	if ((len + u->len) > sizeof(u->u.buffer)) {
+	if (len > sizeof(u->u.buffer) - u->len) {
 		/* On error, dump existing buffer */
 		u->len = 0;
 		rc = -EINVAL;