KVM: x86/mmu: Replace open coded usage of tdp_mmu_page with is_tdp_mmu_page()

Use is_tdp_mmu_page() instead of querying sp->tdp_mmu_page directly so
that all users benefit if KVM ever finds a way to optimize the logic.

No functional change intended.
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20221012181702.3663607-10-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/mmu/mmu.c

@@ -1928,7 +1928,7 @@ static bool is_obsolete_sp(struct kvm *kvm, struct kvm_mmu_page *sp)
 		return true;
 
 	/* TDP MMU pages do not use the MMU generation. */
-	return !sp->tdp_mmu_page &&
+	return !is_tdp_mmu_page(sp) &&
 	       unlikely(sp->mmu_valid_gen != kvm->arch.mmu_valid_gen);
 }
 

arch/x86/kvm/mmu/tdp_mmu.c

@@ -133,7 +133,7 @@ void kvm_tdp_mmu_put_root(struct kvm *kvm, struct kvm_mmu_page *root,
 	if (!refcount_dec_and_test(&root->tdp_mmu_root_count))
 		return;
 
-	WARN_ON(!root->tdp_mmu_page);
+	WARN_ON(!is_tdp_mmu_page(root));
 
 	/*
 	 * The root now has refcount=0.  It is valid, but readers already