vmscan: respect higher order in zone_reclaim()

During page allocation, there are two stages of direct reclaim that are
applied to each zone in the preferred list.  The first stage using
zone_reclaim() reclaims unmapped file backed pages and slab pages if over
defined limits as these are cheaper to reclaim.  The caller specifies the
order of the target allocation but the scan control is not being correctly
initialised.

The impact is that the correct number of pages are being reclaimed but
that lumpy reclaim is not being applied.  This increases the chances of a
full direct reclaim via try_to_free_pages() is required.

This patch initialises the order field of the scan control as requested by
the caller.

[mel@csn.ul.ie: rewrote changelog]
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Mel Gorman <mel@csn.ul.ie>
Cc: Rik van Riel <riel@redhat.com>
Cc: Andy Whitcroft <apw@shadowen.org>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>