KVM: avoid unnecessary wait for a async pf

In current code, it checks async pf completion out of the wait context,
like this:

if (vcpu->arch.mp_state == KVM_MP_STATE_RUNNABLE &&
		    !vcpu->arch.apf.halted)
			r = vcpu_enter_guest(vcpu);
		else {
			......
			kvm_vcpu_block(vcpu)
			 ^- waiting until 'async_pf.done' is not empty
}

kvm_check_async_pf_completion(vcpu)
 ^- delete list from async_pf.done

So, if we check aysnc pf completion first, it can be blocked at
kvm_vcpu_block

Fixed by mark the vcpu is unhalted in kvm_check_async_pf_completion()
path
Signed-off-by: Xiao Guangrong <xiaoguangrong@cn.fujitsu.com>
Acked-by: Gleb Natapov <gleb@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

arch/x86/kvm/x86.c

@@ -6283,6 +6283,7 @@ void kvm_arch_async_page_present(struct kvm_vcpu *vcpu,
 		vcpu->arch.fault.address = work->arch.token;
 		kvm_inject_page_fault(vcpu);
 	}
+	vcpu->arch.apf.halted = false;
 }
 
 bool kvm_arch_can_inject_async_page_present(struct kvm_vcpu *vcpu)