cpumask: use cpu_online in kernel/perf_event.c

Also, we want to check against nr_cpu_ids, not num_possible_cpus().
The latter works, but the correct bounds check is < nr_cpu_ids.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
To: Thomas Gleixner <tglx@linutronix.de>

kernel/perf_event.c

@@ -1614,7 +1614,7 @@ static struct perf_event_context *find_get_context(pid_t pid, int cpu)
 		 * offline CPU and activate it when the CPU comes up, but
 		 * that's for later.
 		 */
-		if (!cpu_isset(cpu, cpu_online_map))
+		if (!cpu_online(cpu))
 			return ERR_PTR(-ENODEV);
 
 		cpuctx = &per_cpu(perf_cpu_context, cpu);