vmscan: shrink_slab() requires the number of lru_pages, not the page order

Presently shrink_slab() has the following scanning equation.

                            lru_scanned        max_pass
  basic_scan_objects = 4 x -------------  x -----------------------------
                            lru_pages        shrinker->seeks (default:2)

  scan_objects = min(basic_scan_objects, max_pass * 2)

If we pass very small value as lru_pages instead real number of lru pages,
shrink_slab() drop much objects rather than necessary.  And now,
__zone_reclaim() pass 'order' as lru_pages by mistake.  That produces a
bad result.

For example, if we receive very low memory pressure (scan = 32, order =
0), shrink_slab() via zone_reclaim() always drop _all_ icache/dcache
objects.  (see above equation, very small lru_pages make very big
scan_objects result).

This patch fixes it.

[akpm@linux-foundation.org: fix layout, typos]
Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Reviewed-by: Minchan Kim <minchan.kim@gmail.com>
Acked-by: Christoph Lameter <cl@linux-foundation.org>
Acked-by: Rik van Riel <riel@redhat.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>