sbin/hammer: Properly break from cache flush loop

sbin/hammer: Properly break from cache flush loop

- hammer_cache_flush() needs to keep track of the first element
  that was pushed back to tail of the list (ref!=0), and break from
  the loop if it hits that element for the second time.

- It's currently working without above check because it breaks from
  the loop after it has accomplished 'target'. If this function was
  changed to release unlimited number of cache then it'll loop
  forever unless tracking those that can't be released (ref!=0).