x86: disable IRQs before doing anything on nmi_shootdown_cpus()

Impact: make nmi_shootdown_cpus() callable from preemptible context

We need to know on which CPU we are running on, and we don't want to be
preempted while doing this.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>