[PATCH] mm: page_state comment more

Clarify that preemption needs to be guarded against with the
__xxx_page_state functions.

Signed-off-by: Nick Piggin <npiggin@suse.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>