mm/vmstat.c: walk the zone in pageblock_nr_pages steps

mm/vmstat.c: walk the zone in pageblock_nr_pages steps

when walking the zone, we can happens to the holes. we should not
align MAX_ORDER_NR_PAGES, so it can skip the normal memory.

In addition, pagetypeinfo_showmixedcount_print reflect fragmentization.
we hope to get more accurate data. therefore, I decide to fix it.

Link: http://lkml.kernel.org/r/1469502526-24486-2-git-send-email-zhongjiang@huawei.com
Signed-off-by: zhong jiang <zhongjiang@huawei.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>