memcg: reduce lock contention v3

This patch series is for reducing memcg's lock contention on res_counter, v3.
(sending today just for reporting current status in my stack.)

It's reported that memcg's res_counter can cause heavy false sharing / lock
conention and scalability is not good. This is for relaxing that.
No terrible bugs are found, I'll maintain/update this until the end of next
merge window. Tests on big-smp and new-good-idea are welcome.

This patch is on to mmotm+Nishimura's fix + Hugh's get_user_pages() patch.
But can be applied directly against mmotm, I think.

numbers:

I used 8cpu x86-64 box and run make -j 12 kernel.
Before make, make clean and drop_caches.

[Before patch(mmotm)] 3 runs
real    3m1.127s
user    4m42.143s
sys     6m22.588s

real    3m0.942s
user    4m42.377s
sys     6m24.463s

real    2m53.982s
user    4m42.635s
sys     6m23.124s

[After patch] 3 runs.
real    2m53.052s
user    4m48.095s
sys     5m43.042s

real    2m54.367s
user    4m43.738s
sys     5m40.626s

real    2m55.108s
user    4m43.367s
sys     5m40.265s


you can see 'sys' is reduced.

Thanks,
-Kame

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