mm/page_alloc: return 0 in case this node has no page within the zone

The whole memory space is divided into several zones and nodes may have no
page in some zones.  In this case, the __absent_pages_in_range() would
return 0, since the range it is searching for is an empty range.

Also this happens more often to those nodes with higher memory range when
there are more nodes, which is a trend for future architectures.

This patch checks the zone range after clamp and adjustment, return 0 if
the range is an empty range.

Link: http://lkml.kernel.org/r/20170206154314.15705-1-richard.weiyang@gmail.com


Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Michal Hocko <mhocko@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>