[PATCH] collect page_states only from online cpus

I noticed the function __read_page_state() curiously high in a q-tools
profile of a write to a software raid0 device.  Seems this is because we're
checking page_states for all possible cpus and we have NR_CPUS possible
when CONFIG_HOTPLUG_CPU=y.  The default config for ia64 is now NR_CPUS=512,
so on a little 8-way box, this is a significant waste of time.  The patch
below updates __read_page_state() and __get_page_state() to only count
page_state info for online cpus.  To keep the stats consistent, the
page_alloc notifier is updated to move page_states off of the cpu going
offline.  On my profile, this dropped __read_page_state() back into the
noise and boosted block write performance by 5% (as measured by spew -
http://spew.berlios.de).

Signed-off-by: Alex Williamson <alex.williamson@hp.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

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