vhost: vringh: Modify the return value check

The return value of copy_from_iter and copy_to_iter can't be negative,
check whether the copied lengths are equal.

Fixes: 309bba39 ("vringh: iterate on iotlb_translate to handle large translations")
Cc: "Stefano Garzarella" <sgarzare@redhat.com>
Signed-off-by: zhang jiao <zhangjiao2@cmss.chinamobile.com>
Message-Id: <20250910091739.2999-1-zhangjiao2@cmss.chinamobile.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>