percpu: fix percpu accessors to potentially !cpu_possible() cpus: pnpbios

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: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Mike Travis <travis@sgi.com>
Acked-by: Ingo Molnar <mingo@elte.hu>
Cc: Adam Belay <ambx1@neo.rr.com>