The use of blocking notifier by _cpu_up and _cpu_down in cpu.c has two
problem.

1/ An interaction with the workqueue notifier causes lockdep to spit a
   warning.

2/ A notifier could conceivable be added or removed while _cpu_up or
   _cpu_down are in process.  As each notifier is called twice (prepare
   then commit/abort) this could be unhealthy.

To fix to we simply take cpu_add_remove_lock while adding or removing
notifiers to/from the list.

This makes the 'blocking' usage unnecessary as all accesses to cpu_chain
are now protected by cpu_add_remove_lock.  So change "blocking" to "raw" in
all relevant places.  This fixes 1.

Credit: Andrew Morton
Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: Michal Piotrowski <michal.k.k.piotrowski@gmail.com> (reporter)
Signed-off-by: Neil Brown <neilb@suse.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>