perf_counter, x86: protect per-cpu variables with compile barriers only

Per-cpu variables needn't to be protected with cpu barriers
(smp_wmb()). Protection is only needed for preemption on the same cpu
(rescheduling or the nmi handler). This can be done using a compiler
barrier only.

[ Impact: micro-optimization ]

Signed-off-by: Robert Richter <robert.richter@amd.com>
Cc: Paul Mackerras <paulus@samba.org>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <1241002046-8832-6-git-send-email-robert.richter@amd.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>