x86/apic/x2apic, smp/hotplug: Don't use before alloc in x2apic_cluster_probe()

I made a mistake while converting the driver to the hotplug state
machine and as a result x2apic_cluster_probe() was accessing
cpus_in_cluster before allocating it.

This patch fixes it by setting the cpumask after the allocation the
memory succeeded.

While at it, I marked two functions static which are only used within
this file.

Reported-by: Laura Abbott <labbott@redhat.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Fixes: 6b2c28471de5 ("x86/x2apic: Convert to CPU hotplug state machine")
Link: http://lkml.kernel.org/r/1470924515-9444-1-git-send-email-bigeasy@linutronix.de
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/kernel/apic/x2apic_cluster.c

@@ -155,7 +155,7 @@ static void init_x2apic_ldr(void)
 /*
  * At CPU state changes, update the x2apic cluster sibling info.
  */
-int x2apic_prepare_cpu(unsigned int cpu)
+static int x2apic_prepare_cpu(unsigned int cpu)
 {
 	if (!zalloc_cpumask_var(&per_cpu(cpus_in_cluster, cpu), GFP_KERNEL))
 		return -ENOMEM;
@@ -168,7 +168,7 @@ int x2apic_prepare_cpu(unsigned int cpu)
 	return 0;
 }
 
-int x2apic_dead_cpu(unsigned int this_cpu)
+static int x2apic_dead_cpu(unsigned int this_cpu)
 {
 	int cpu;
 
@@ -186,13 +186,18 @@ int x2apic_dead_cpu(unsigned int this_cpu)
 static int x2apic_cluster_probe(void)
 {
 	int cpu = smp_processor_id();
+	int ret;
 
 	if (!x2apic_mode)
 		return 0;
 
+	ret = cpuhp_setup_state(CPUHP_X2APIC_PREPARE, "X2APIC_PREPARE",
+				x2apic_prepare_cpu, x2apic_dead_cpu);
+	if (ret < 0) {
+		pr_err("Failed to register X2APIC_PREPARE\n");
+		return 0;
+	}
 	cpumask_set_cpu(cpu, per_cpu(cpus_in_cluster, cpu));
-	cpuhp_setup_state(CPUHP_X2APIC_PREPARE, "X2APIC_PREPARE",
-			  x2apic_prepare_cpu, x2apic_dead_cpu);
 	return 1;
 }