KVM: Convert gfn_to_page() to use kvm_follow_pfn()

Convert gfn_to_page() to the new kvm_follow_pfn() internal API, which will
eventually allow removing gfn_to_pfn() and kvm_pfn_to_refcounted_page().

Tested-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Tested-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <20241010182427.1434605-76-seanjc@google.com>

Author: sean-jc

virt/kvm/kvm_main.c

@@ -3139,14 +3139,16 @@ EXPORT_SYMBOL_GPL(kvm_prefetch_pages);
  */
 struct page *gfn_to_page(struct kvm *kvm, gfn_t gfn)
 {
-	kvm_pfn_t pfn;
-
-	pfn = gfn_to_pfn(kvm, gfn);
-
-	if (is_error_noslot_pfn(pfn))
-		return NULL;
+	struct page *refcounted_page = NULL;
+	struct kvm_follow_pfn kfp = {
+		.slot = gfn_to_memslot(kvm, gfn),
+		.gfn = gfn,
+		.flags = FOLL_WRITE,
+		.refcounted_page = &refcounted_page,
+	};
 
-	return kvm_pfn_to_refcounted_page(pfn);
+	(void)kvm_follow_pfn(&kfp);
+	return refcounted_page;
 }
 EXPORT_SYMBOL_GPL(gfn_to_page);