memcg numa node scan update.

In the last month, I added round-robin scan of numa nodes at
hittling limit, and wrote "a better algorithm is needed."

Here is update. Because some of patches are bugfixes, I may
cut out them as independent patch.

Pathc 6-7/7 implements a selection logic.

==
Tested on 8cpu/24GB system, which has 2 nodes.
limit memory to 300MB and run httpd under it.
httpd's working set is 4096files/600MB.

Then, do 40960 access by apache-bench. and see how memory reclaim costs.
Because a thread of httpd doesn't consume cpu much, the number of
working threads are not balanced between numa nodes and file caches
will be not balanced.

[round-robin]
 [kamezawa@bluextal ~]$ cat /cgroup/memory/test/memory.scan_stat
  scanned_pages_by_limit 550740
  freed_pages_by_limit 206473
  elapsed_ns_by_limit 9485418834

[After patch]
  scanned_pages_by_limit 521520
  freed_pages_by_limit 199330
  elapsed_ns_by_limit 7904913234

I can see elapsed time is decreased.
Test on big machine is welcomed.

Thanks,
-Kame

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