core: Clean up user return notifers use of per_cpu

Instead of using per_cpu(..., raw_smp_processor_id()), use
__get_cpu_var(...).

Signed-off-by: Avi Kivity <avi@redhat.com>
LKML-Reference: <1259578491-4589-1-git-send-email-avi@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>