kvm: x86: Fix error handling in the function kvm_lapic_sync_from_vapic

This fixes error handling in the function kvm_lapic_sync_from_vapic
by checking if the call to kvm_read_guest_cached has returned a
error code to signal to its caller the call to this function has
failed and due to this we must immediately return to the caller
of kvm_lapic_sync_from_vapic to avoid incorrectly call apic_set_tpc
if a error has occurred here.

Signed-off-by: Nicholas Krause <xerofoify@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/lapic.c

@@ -1900,8 +1900,9 @@ void kvm_lapic_sync_from_vapic(struct kvm_vcpu *vcpu)
 	if (!test_bit(KVM_APIC_CHECK_VAPIC, &vcpu->arch.apic_attention))
 		return;
 
-	kvm_read_guest_cached(vcpu->kvm, &vcpu->arch.apic->vapic_cache, &data,
-				sizeof(u32));
+	if (kvm_read_guest_cached(vcpu->kvm, &vcpu->arch.apic->vapic_cache, &data,
+				  sizeof(u32)))
+		return;
 
 	apic_set_tpr(vcpu->arch.apic, data & 0xff);
 }