When devpts is unmounted, there may be a no-longer-used IDR tree hanging
off the superblock we are about to kill. This needs to be cleaned up
before destroying the SB.
The leak is usually not a big deal because unmounting devpts is typically
done when shutting down the whole machine. However, shutting down an LXC
container instead of a physical machine exposes the problem (the garbage
is detectable with kmemleak).