Register a cpu hotplug notifier which reinitializes the scheduler domains
hierarchy.  The notifier temporarily attaches all running cpus to a "dummy"
domain (like we currently do during boot) to avoid balancing.  It then calls
arch_init_sched_domains which rebuilds the "real" domains and reattaches the
cpus to them.

Also change __init attributes to __devinit where necessary.
Signed-off-by: Nathan Lynch <nathanl@austin.ibm.com>

Alterations from Nick Piggin:

* Detach all domains in CPU_UP|DOWN_PREPARE notifiers. Reinitialise and
  reattach in CPU_ONLINE|DEAD|UP_CANCELED. This ensures the domains as
  seen from the scheduler won't become out of synch with the cpu_online_map.

* This allows us to remove runtime cpu_online verifications. Do that.

* Dummy domains are __devinitdata.

* Remove the hackery in arch_init_sched_domains to work around the fact that
  the domains used to work with cpu_possible maps, but node_to_cpumask returned
  a cpu_online map.
Signed-off-by: Nick Piggin <nickpiggin@yahoo.com.au>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>