xen/gntdevt: Fix race condition in gntdev_release()

While gntdev_release() is called the MMU notifier is still registered
and can traverse priv->maps list even if no pages are mapped (which is
the case -- gntdev_release() is called after all). But
gntdev_release() will clear that list, so make sure that only one of
those things happens at the same time.
Signed-off-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>

drivers/xen/gntdev.c

@@ -568,12 +568,14 @@ static int gntdev_release(struct inode *inode, struct file *flip)
 
 	pr_debug("priv %p\n", priv);
 
+	mutex_lock(&priv->lock);
 	while (!list_empty(&priv->maps)) {
 		map = list_entry(priv->maps.next, struct grant_map, next);
 		list_del(&map->next);
 		gntdev_put_map(NULL /* already removed */, map);
 	}
 	WARN_ON(!list_empty(&priv->freeable_maps));
+	mutex_unlock(&priv->lock);
 
 	if (use_ptemod)
 		mmu_notifier_unregister(&priv->mn, priv->mm);