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>