KVM: nVMX: Cast exit_reason to u16 to check for nested EXTERNAL_INTERRUPT

Explicitly check only the basic exit reason when emulating an external
interrupt VM-Exit in nested_vmx_vmexit().  Checking the full exit reason
doesn't currently cause problems, but only because the only exit reason
modifier support by KVM is FAILED_VMENTRY, which is mutually exclusive
with EXTERNAL_INTERRUPT.  Future modifiers, e.g. ENCLAVE_MODE, will
coexist with EXTERNAL_INTERRUPT.
Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Message-Id: <20200415175519.14230-9-sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/vmx/nested.c

@@ -4406,7 +4406,7 @@ void nested_vmx_vmexit(struct kvm_vcpu *vcpu, u32 exit_reason,
 	vcpu->arch.mp_state = KVM_MP_STATE_RUNNABLE;
 
 	if (likely(!vmx->fail)) {
-		if (exit_reason == EXIT_REASON_EXTERNAL_INTERRUPT &&
+		if ((u16)exit_reason == EXIT_REASON_EXTERNAL_INTERRUPT &&
 		    nested_exit_intr_ack_set(vcpu)) {
 			int irq = kvm_cpu_get_interrupt(vcpu);
 			WARN_ON(irq < 0);