pk: Avoid unnecessary cleaner thread wakeups
pk: Avoid unnecessary cleaner thread wakeups

- Don't wake up the cache_entry cleaner thread to have it recalculate its
next wakeup time when that time will be later than the one currently
scheduled.  This is redundant and can happen several times in a row,
causing more wakeups than would occur otherwise.

cmusatyalab/openisr,cmusatyalab/openisr,cmusatyalab/openisr,cmusatyalab/openisr,cmusatyalab/openisr