mm: vmscan: check if kswapd should writepage once per pgdat scan

Currently kswapd checks if it should start writepage as it shrinks each
zone without taking into consideration if the zone is balanced or not.
This is not wrong as such but it does not make much sense either.  This
patch checks once per pgdat scan if kswapd should be writing pages.

Signed-off-by: Mel Gorman <mgorman@suse.de>
Reviewed-by: Michal Hocko <mhocko@suse.cz>
Acked-by: Rik van Riel <riel@redhat.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Jiri Slaby <jslaby@suse.cz>
Cc: Valdis Kletnieks <Valdis.Kletnieks@vt.edu>
Tested-by: Zlatko Calusic <zcalusic@bitsync.net>
Cc: dormando <dormando@rydia.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Git-commit: b7ea3c417b6c2e74ca1cb051568f60377908928d
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


[vinmenon@codeaurora.org: resolve trivial merge conflicts]
Change-Id: I7cb0fb685f8346f07d0fc4810f6c593334cd1590
Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>