[RFC] use radix_tree for non-resident page tracking

[WU Fengguang]

Hi Rik,
The CLOCK-Pro page replacement is quite appealing, and I'd like to
contribute an idea: How about store bookkeeping info of dropped pages
in-place in radix_tree?

The slots in radix_tree_node can be used for bookkeeping data when
the corresponding pages are dropped. When all pages in a radix_tree_node
have been dropped, it is registered in an array/list for delayed
reclaim.

It would be fast and simple:
- no cache-line pollution
- no extra lock (with Nick Piggin's great RCU improvement)
- ready to use lookup code

The memory footprint should be roughly the same, though with a whole
word of space for each page ;)

It can not cover the swap space, which should be handled differently
anyway, I guess.

Regards,
Wu Fengguang
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