towards I/O aware memory cgroup

From a view of patch management, this set is a mixture of a few features for
memcg, and I should divide them to some groups. But, at first, I'd like to
show the total view. This set is consists from 5 sets. Main purpose is
create a room in page_cgroup for I/O tracking and add light-weight access method
for file-cache related accounting. 

1.   An virtual-indexed array.
2,3. Use virtual-indexed array for id-to-memory_cgroup detection.
4.   modify page_cgroup to use ID instead of pointer, this gives us enough
     spaces for further memory tracking.
5,6   Use light-weight locking mechanism for file related accounting.
7.   use spin_lock instead of bit_spinlock.


As a function,  patch 5,6 can be an independent patch and I'll accept
reordering series of patch if someone requests.
But we'll need all, I think.
(irq_save for patch 7 will be required later.)

Any comments are welcome.

Thanks,
-Kame 

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