perf_counter: Fix atomic_set vs. atomic64_t type mismatch

Using atomic_set on an atomic64_t variable gives a compiler
warning on powerpc, and won't give the desired result at runtime.
This fixes an instance of this error in the perf_counter code.
Signed-off-by: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <18995.20490.979429.244883@cargo.ozlabs.ibm.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/perf_counter.c

@@ -1283,7 +1283,7 @@ static void perf_ctx_adjust_freq(struct perf_counter_context *ctx)
 		if (!interrupts) {
 			perf_disable();
 			counter->pmu->disable(counter);
-			atomic_set(&hwc->period_left, 0);
+			atomic64_set(&hwc->period_left, 0);
 			counter->pmu->enable(counter);
 			perf_enable();
 		}