devpts: plug the memory leak in kill_sb

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).

Signed-off-by: Ilija Hadzic <ihadzic@research.bell-labs.com>
Cc: Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>