powerpc/smp: remove call to ipi_call_lock()/ipi_call_unlock()

1) call_function.lock used in smp_call_function_many() is just to protect
   call_function.queue and &data->refs, cpu_online_mask is outside of the
   lock. And it's not necessary to protect cpu_online_mask,
   because data->cpumask is pre-calculate and even if a cpu is brougt up
   when calling arch_send_call_function_ipi_mask(), it's harmless because
   validation test in generic_smp_call_function_interrupt() will take care
   of it.

2) For cpu down issue, stop_machine() will guarantee that no concurrent
   smp_call_fuction() is processing.

Signed-off-by: Yong Zhang <yong.zhang0@gmail.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>