[S390] show_interrupts: prevent cpu hotplug when walking cpu_online_map.

Surround all the code withing show_interrupts() with
get/put_online_cpus() to prevent strange results wrt cpu hotplug.

Signed-off-by: Heiko Carstens <[e-mail]>
Signed-off-by: Martin Schwidefsky <[e-mail]>

arch/s390/kernel/irq.c

···
 	static const char *intrclass_names[] = { "EXT", "I/O", };
 	int i = *(loff_t *) v, j;
 
+	get_online_cpus();
 	if (i == 0) {
 		seq_puts(p, "           ");
 		for_each_online_cpu(j)
···
                 seq_putc(p, '\n');
 
         }
-
+	put_online_cpus();
         return 0;
 }