Pretty simple.

- use a timer to kick off a pdflush thread every five seconds
  to run the kupdate code.

- wakeup_bdflush() kicks off a pdflush thread to run the current
  bdflush function.

There's some loss of functionality here - the ability to tune
the writeback periods.  The numbers are hardwired at present.
But the intent is that buffer-based writeback disappears
altogether.  New mechanisms for tuning the writeback will
need to be introduced.