x86: nmi_32.c cleanup - use for_each_online_cpu helper

Since cpu_online_map is touched (by for_each_online_cpu)
at moment when cpu_callin_map is already filled up we can
get rid of its checking at all

Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>
Cc: hpa@zytor.com
Cc: mingo@redhat.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>