kvm: fix waitqueue_active without memory barrier in virt/kvm/async_pf.c

kvm: fix waitqueue_active without memory barrier in virt/kvm/async_pf.c

async_pf_execute() seems to be missing a memory barrier which might
cause the waker to not notice the waiter and miss sending a wake_up as
in the following figure.

        async_pf_execute                    kvm_vcpu_block
------------------------------------------------------------------------
spin_lock(&vcpu->async_pf.lock);
if (waitqueue_active(&vcpu->wq))
/* The CPU might reorder the test for
   the waitqueue up here, before
   prior writes complete */
                                    prepare_to_wait(&vcpu->wq, &wait,
                                      TASK_INTERRUPTIBLE);
                                    /*if (kvm_vcpu_check_block(vcpu) < 0) */
                                     /*if (kvm_arch_vcpu_runnable(vcpu)) { */
                                      ...
                                      return (vcpu->arch.mp_state == KVM_MP_STATE_RUNNABLE &&
                                        !vcpu->arch.apf.halted)
                                        || !list_empty_careful(&vcpu->async_pf.done)
                                     ...
                                     return 0;
list_add_tail(&apf->link,
  &vcpu->async_pf.done);
spin_unlock(&vcpu->async_pf.lock);
                                    waited = true;
                                    schedule();
------------------------------------------------------------------------

The attached patch adds the missing memory barrier.

I found this issue when I was looking through the linux source code
for places calling waitqueue_active() before wake_up*(), but without
preceding memory barriers, after sending a patch to fix a similar
issue in drivers/tty/n_tty.c  (Details about the original issue can be
found here: https://lkml.org/lkml/2015/9/28/849).

Signed-off-by: Kosuke Tatsukawa <tatsu@ab.jp.nec.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>