virtio_balloon: Use outer variable 'page'

There is no need to define a local variable 'page',
just use outer variable 'page'.

Signed-off-by: zhang jiao <zhangjiao2@cmss.chinamobile.com>
Message-Id: <20241120054920.35291-1-zhangjiao2@cmss.chinamobile.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Gavin Shan <gshan@redhat.com>