nfs: nfs_kill_super() should call bdi_unregister() after killing super

Otherwise we could be attempting to flush data for a writeback
thread and bdi that have already disappeared.
Signed-off-by: NJens Axboe <jens.axboe@oracle.com>

fs/nfs/super.c

@@ -2190,8 +2190,8 @@ static void nfs_kill_super(struct super_block *s)
 {
 	struct nfs_server *server = NFS_SB(s);
 
-	bdi_unregister(&server->backing_dev_info);
 	kill_anon_super(s);
+	bdi_unregister(&server->backing_dev_info);
 	nfs_fscache_release_super_cookie(s);
 	nfs_free_server(server);
 }