[PATCH] debug check in put_page_testzero()

As suggested by Daniel - it's a bug to run put_page_testzero
against a zero-ref page.

include/linux/mm.h

@@ -195,7 +195,11 @@ struct page {
  */
 #define get_page(p)		atomic_inc(&(p)->count)
 #define __put_page(p)		atomic_dec(&(p)->count)
-#define put_page_testzero(p) 	atomic_dec_and_test(&(p)->count)
+#define put_page_testzero(p)				\
+	({						\
+		BUG_ON(page_count(page) == 0);		\
+		atomic_dec_and_test(&(p)->count);	\
+	})
 #define page_count(p)		atomic_read(&(p)->count)
 #define set_page_count(p,v) 	atomic_set(&(p)->count, v)
 extern void FASTCALL(__page_cache_release(struct page *));