KVM: use is_idle_task() instead of idle_cpu() to decide when to halt in async_pf

As Frederic pointed idle_cpu() may return false even if async fault
happened in the idle task if wake up is pending. In this case the code
will try to put idle task to sleep. Fix this by using is_idle_task() to
check for idle task.
Reported-by: NFrederic Weisbecker <fweisbec@gmail.com>
Signed-off-by: NGleb Natapov <gleb@redhat.com>
Signed-off-by: NMarcelo Tosatti <mtosatti@redhat.com>

arch/x86/kernel/kvm.c

@@ -120,11 +120,6 @@ void kvm_async_pf_task_wait(u32 token)
 	struct kvm_task_sleep_head *b = &async_pf_sleepers[key];
 	struct kvm_task_sleep_node n, *e;
 	DEFINE_WAIT(wait);
-	int cpu, idle;
-
-	cpu = get_cpu();
-	idle = idle_cpu(cpu);
-	put_cpu();
 
 	spin_lock(&b->lock);
 	e = _find_apf_task(b, token);
@@ -138,7 +133,7 @@ void kvm_async_pf_task_wait(u32 token)
 
 	n.token = token;
 	n.cpu = smp_processor_id();
-	n.halted = idle || preempt_count() > 1;
+	n.halted = is_idle_task(current) || preempt_count() > 1;
 	init_waitqueue_head(&n.wq);
 	hlist_add_head(&n.link, &b->list);
 	spin_unlock(&b->lock);