struct page_state is large (148 bytes) and we put them on the stack in awkward
code paths (page reclaim...)

So implement a simple read_page_state() which can be used to pluck out a
single member from the all-cpus page_state accumulators.
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>