x86/apic: Use dead_cpu instead of current CPU when cleaning up

x2apic_dead_cpu() cleans up the leftovers of a CPU which got unplugged, but
instead of clearing the dead cpu bit in the cluster mask it clears the
current (alive) cpu bit. Noticed because smp_processor_id() is called in
preemptible code and triggers a debug warning.

[ tglx: Rewrote changelog ]

Fixes: 023a611748fd ("x86/apic/x2apic: Simplify cluster management")
Signed-off-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20170926170845.13955-1-bp@alien8.de

arch/x86/kernel/apic/x2apic_cluster.c

@@ -156,7 +156,7 @@ static int x2apic_dead_cpu(unsigned int dead_cpu)
 {
 	struct cluster_mask *cmsk = per_cpu(cluster_masks, dead_cpu);
 
-	cpumask_clear_cpu(smp_processor_id(), &cmsk->mask);
+	cpumask_clear_cpu(dead_cpu, &cmsk->mask);
 	free_cpumask_var(per_cpu(ipi_mask, dead_cpu));
 	return 0;
 }