virtio_balloon: fix race by fill and leak

During my compaction-related stuff, I encountered a bug
with ballooning.

With repeated inflating and deflating cycle, guest memory(
ie, cat /proc/meminfo | grep MemTotal) is decreased and
couldn't be recovered.

The reason is balloon_lock doesn't cover release_pages_balloon
so struct virtio_balloon fields could be overwritten by race
of fill_balloon(e,g, vb->*pfns could be critical).

This patch fixes it in my test.

Cc: <stable@vger.kernel.org>
Signed-off-by: Minchan Kim <minchan@kernel.org>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

drivers/virtio/virtio_balloon.c

@@ -209,8 +209,8 @@ static unsigned leak_balloon(struct virtio_balloon *vb, size_t num)
 	 */
 	if (vb->num_pfns != 0)
 		tell_host(vb, vb->deflate_vq);
-	mutex_unlock(&vb->balloon_lock);
 	release_pages_balloon(vb);
+	mutex_unlock(&vb->balloon_lock);
 	return num_freed_pages;
 }