KVM: Use page_private()/set_page_private() apis

Besides using an established api, this allows using kvm in older kernels.

Signed-off-by: Markus Rechberger <markus.rechberger@amd.com>
Signed-off-by: Avi Kivity <avi@qumranet.com>
diff --git a/drivers/kvm/kvm.h b/drivers/kvm/kvm.h
index 04574a9..9a49b2e 100644
--- a/drivers/kvm/kvm.h
+++ b/drivers/kvm/kvm.h
@@ -523,7 +523,7 @@
 {
 	struct page *page = pfn_to_page(shadow_page >> PAGE_SHIFT);
 
-	return (struct kvm_mmu_page *)page->private;
+	return (struct kvm_mmu_page *)page_private(page);
 }
 
 static inline u16 read_fs(void)