nr_dirty and nr_congested are increased only when the page is dirty.  So
if all pages are clean, both them will be zero.  In this case, we should
not mark the zone congested.
Signed-off-by: NShaohua Li <shaohua.li@intel.com>
Reviewed-by: NJohannes Weiner <hannes@cmpxchg.org>
Reviewed-by: NMinchan Kim <minchan.kim@gmail.com>
Acked-by: NMel Gorman <mel@csn.ul.ie>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>