[PATCH] for_each_possible_cpu: defines for_each_possible_cpu

for_each_cpu() is a for-loop over cpu_possible_map.  for_each_online_cpu is
for-loop cpu over cpu_online_map.  .....for_each_cpu() is not sufficiently
explicit and can lead to mistakes.

This patch adds for_each_possible_cpu() in preparation for the removal of
for_each_cpu().

Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>