s390/smp: disallow CPU hotplug of CPU 0

On s390, CPU 0 has special properties in comparison to other CPUs, as it
cannot be deconfigured for example. Therefore, allowing to hotplug CPU 0
introduces additional complexity when handling these properties.
Disallowing to hotplug CPU 0 allows to remove such complexities.

This follows x86 which also prevents offlining of CPU0 since commit
e59e74dc ("x86/topology: Remove CPU0 hotplug option").

[hca@linux.ibm.com: changed commit message]
Suggested-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Tobias Huschle <huschle@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

arch/s390/kernel/smp.c

@@ -1076,11 +1076,9 @@ static ssize_t cpu_configure_store(struct device *dev,
 	cpus_read_lock();
 	mutex_lock(&smp_cpu_state_mutex);
 	rc = -EBUSY;
-	/* disallow configuration changes of online cpus and cpu 0 */
+	/* disallow configuration changes of online cpus */
 	cpu = dev->id;
 	cpu = smp_get_base_cpu(cpu);
-	if (cpu == 0)
-		goto out;
 	for (i = 0; i <= smp_cpu_mtid; i++)
 		if (cpu_online(cpu + i))
 			goto out;
@@ -1180,7 +1178,7 @@ static int smp_add_present_cpu(int cpu)
 		return -ENOMEM;
 	per_cpu(cpu_device, cpu) = c;
 	s = &c->dev;
-	c->hotpluggable = 1;
+	c->hotpluggable = !!cpu;
 	rc = register_cpu(c, cpu);
 	if (rc)
 		goto out;