This /proc tunable sets the kupdate interval.  It has a couple of problems:

- No way to turn it off completely (userspace dirty memory management
  solutions require this).

- If it has been set to one hour and then the user resets it to five
  seconds, that resetting will not take effect for up to an hour.

Fix that up by providing a sysctl handler.  Setting the tunable to zero now
disables the kupdate function.