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

arch/x86/kernel/reboot.c

@@ -577,6 +577,7 @@ static struct notifier_block crash_nmi_nb = {
 void nmi_shootdown_cpus(nmi_shootdown_cb callback)
 {
 	unsigned long msecs;
+	local_irq_disable();
 
 	/* Make a note of crashing cpu. Will be used in NMI callback.*/
 	crashing_cpu = safe_smp_processor_id();