xen/xenbus: fix overflow check in xenbus_file_write()

Acked-by: Ian Campbell <[e-mail]>
Reported-by: Dan Carpenter <[e-mail]>
Signed-off-by: Jan Beulich <[e-mail]>
[v1: Rebased on upstream]
Signed-off-by: Konrad Rzeszutek Wilk <[e-mail]>

drivers/xen/xenbus/xenbus_dev_frontend.c

···
 		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;