Impact: CPU iterator bugfixes

Percpu areas are only allocated for possible cpus.  In general, you
shouldn't access random cpu's percpu areas.
Signed-off-by: NRusty Russell <rusty@rustcorp.com.au>
Signed-off-by: NMike Travis <travis@sgi.com>
Acked-by: NIngo Molnar <mingo@elte.hu>
Cc: Adam Belay <ambx1@neo.rr.com>