mm: build_zonelists(): move clear node_load[] to __build_all_zonelists()

If node_load[] is cleared everytime build_zonelists() is
called,node_load[] will have no help to find the next node that should
appear in the given node's fallback list.

Because of the bug, zonelist's node_order is not calculated as expected.
This bug affects on big machine, which has asynmetric node distance.

[synmetric NUMA's node distance]
     0    1    2
0   10   12   12
1   12   10   12
2   12   12   10

[asynmetric NUMA's node distance]
     0    1    2
0   10   12   20
1   12   10   14
2   20   14   10

This (my bug) is very old but no one has reported this for a long time.
Maybe because the number of asynmetric NUMA is very small and they use
cpuset for customizing node memory allocation fallback.

[akpm@linux-foundation.org: fix CONFIG_NUMA=n build]
Signed-off-by: Bo Liu <bo-liu@hotmail.com>
Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Mel Gorman <mel@csn.ul.ie>
Cc: Christoph Lameter <cl@linux-foundation.org>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/page_alloc.c

@@ -2544,7 +2544,6 @@ static void build_zonelists(pg_data_t *pgdat)
 	prev_node = local_node;
 	nodes_clear(used_mask);
 
-	memset(node_load, 0, sizeof(node_load));
 	memset(node_order, 0, sizeof(node_order));
 	j = 0;
 
@@ -2653,6 +2652,9 @@ static int __build_all_zonelists(void *dummy)
 {
 	int nid;
 
+#ifdef CONFIG_NUMA
+	memset(node_load, 0, sizeof(node_load));
+#endif
 	for_each_online_node(nid) {
 		pg_data_t *pgdat = NODE_DATA(nid);