Writeback takes out a lock on the cache block, so will increase the
latency for any concurrent io.

This patch works by placing 2 sentinel objects on each level of the
multiqueues.  Every WRITEBACK_PERIOD the oldest sentinel gets moved to
the newest end of the queue level.

When looking for writeback work:
  if less than 25% of the cache is clean:
    we select the oldest object with the lowest hit count
  otherwise:
    we select the oldest object that is not past a writeback sentinel.
Signed-off-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>