KVM: x86: take slots_lock in kvm_free_pit

kvm_vm_release() did not have slots_lock when calling
kvm_io_bus_unregister_dev() and this went unnoticed until 4a12f951
("KVM: mark kvm->busses as rcu protected") added dynamic checks.
Luckily, there should be no race at that point:

  =============================
  WARNING: suspicious RCU usage
  4.12.0.kvm+ #0 Not tainted
  -----------------------------
  ./include/linux/kvm_host.h:479 suspicious rcu_dereference_check() usage!

   lockdep_rcu_suspicious+0xc5/0x100
   kvm_io_bus_unregister_dev+0x173/0x190 [kvm]
   kvm_free_pit+0x28/0x80 [kvm]
   kvm_arch_sync_events+0x2d/0x30 [kvm]
   kvm_put_kvm+0xa7/0x2a0 [kvm]
   kvm_vm_release+0x21/0x30 [kvm]
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>

arch/x86/kvm/i8254.c

@@ -724,8 +724,10 @@ void kvm_free_pit(struct kvm *kvm)
 	struct kvm_pit *pit = kvm->arch.vpit;
 
 	if (pit) {
+		mutex_lock(&kvm->slots_lock);
 		kvm_io_bus_unregister_dev(kvm, KVM_PIO_BUS, &pit->dev);
 		kvm_io_bus_unregister_dev(kvm, KVM_PIO_BUS, &pit->speaker_dev);
+		mutex_unlock(&kvm->slots_lock);
 		kvm_pit_set_reinject(pit, false);
 		hrtimer_cancel(&pit->pit_state.timer);
 		kthread_destroy_worker(pit->worker);