- /proc/vmstat:pageoutrun and /proc/vmstat:allocstall are always
  identical.  Rework this so that

  - "allocstall" is the number of times a page allocator ran diect reclaim

  - "pageoutrun" is the number of times kswapd ran page reclaim

- Add a new stat: "pgrotated".  The number of pages which were
  rotated to the tail of the LRU for immediate reclaim by
  rotate_reclaimable_page().

- Document things a bit.