ceph: only unregister registered bdi

Signed-off-by: Sage Weil <sage@newdream.net>

fs/ceph/super.c

@@ -942,6 +942,7 @@ static void ceph_kill_sb(struct super_block *s)
 	dout("kill_sb %p\n", s);
 	ceph_mdsc_pre_umount(&client->mdsc);
 	kill_anon_super(s);    /* will call put_super after sb is r/o */
+	if (s->s_bdi == &client->backing_dev_info)
 		bdi_unregister(&client->backing_dev_info);
 	bdi_destroy(&client->backing_dev_info);
 	ceph_destroy_client(client);