f2fs: initialize page->private when using for our internal use

We need to guarantee it's initially zero. Otherwise, it'll hurt entire flag
operations.

Fixes: b763f3b ("f2fs: restructure f2fs page.private layout")
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

Author: Jaegeuk Kim

fs/f2fs/data.c

@@ -3819,6 +3819,8 @@ int f2fs_migrate_page(struct address_space *mapping,
 		get_page(newpage);
 	}
 
+	/* guarantee to start from no stale private field */
+	set_page_private(newpage, 0);
 	if (PagePrivate(page)) {
 		set_page_private(newpage, page_private(page));
 		SetPagePrivate(newpage);

fs/f2fs/f2fs.h

@@ -1331,7 +1331,8 @@ enum {
 #define PAGE_PRIVATE_GET_FUNC(name, flagname) \
 static inline bool page_private_##name(struct page *page) \
 { \
-	return test_bit(PAGE_PRIVATE_NOT_POINTER, &page_private(page)) && \
+	return PagePrivate(page) && \
+		test_bit(PAGE_PRIVATE_NOT_POINTER, &page_private(page)) && \
 		test_bit(PAGE_PRIVATE_##flagname, &page_private(page)); \
 }
 
@@ -1341,6 +1342,7 @@ static inline void set_page_private_##name(struct page *page) \
 	if (!PagePrivate(page)) { \
 		get_page(page); \
 		SetPagePrivate(page); \
+		set_page_private(page, 0); \
 	} \
 	set_bit(PAGE_PRIVATE_NOT_POINTER, &page_private(page)); \
 	set_bit(PAGE_PRIVATE_##flagname, &page_private(page)); \
@@ -1392,6 +1394,7 @@ static inline void set_page_private_data(struct page *page, unsigned long data)
 	if (!PagePrivate(page)) {
 		get_page(page);
 		SetPagePrivate(page);
+		set_page_private(page, 0);
 	}
 	set_bit(PAGE_PRIVATE_NOT_POINTER, &page_private(page));
 	page_private(page) |= data << PAGE_PRIVATE_MAX;