KVM: Handle spurious acks for PIT interrupts

Spurious acks can be generated, for example if the PIC is being reset.
Handle those acks gracefully rather than flooding the log with warnings.
Signed-off-by: Avi Kivity <avi@qumranet.com>

arch/x86/kvm/i8254.c

@@ -228,7 +228,7 @@ void kvm_pit_ack_irq(struct kvm_irq_ack_notifier *kian)
 						 irq_ack_notifier);
 	spin_lock(&ps->inject_lock);
 	if (atomic_dec_return(&ps->pit_timer.pending) < 0)
-		WARN_ON(1);
+		atomic_inc(&ps->pit_timer.pending);
 	ps->irq_ack = 1;
 	spin_unlock(&ps->inject_lock);
 }