KVM: x86/mmu: Use IS_ENABLED() to avoid RETPOLINE for TDP page faults

Use IS_ENABLED() instead of an #ifdef to activate the anti-RETPOLINE fast
path for TDP page faults.  The generated code is identical, and the #ifdef
makes it dangerously difficult to extend the logic (guess who forgot to
add an "else" inside the #ifdef and ran through the page fault handler
twice).

No functional or binary change intented.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20220423034752.1161007-9-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/mmu/mmu_internal.h

@@ -248,10 +248,10 @@ static inline int kvm_mmu_do_page_fault(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa,
 		.req_level = PG_LEVEL_4K,
 		.goal_level = PG_LEVEL_4K,
 	};
-#ifdef CONFIG_RETPOLINE
-	if (fault.is_tdp)
+
+	if (IS_ENABLED(CONFIG_RETPOLINE) && fault.is_tdp)
 		return kvm_tdp_page_fault(vcpu, &fault);
-#endif
+
 	return vcpu->arch.mmu->page_fault(vcpu, &fault);
 }