f2fs: check truncation of mapping after lock_page

We call lock_page when we need to update a page after readpage.
Between grab and lock page, the page can be truncated by other thread.
So, we should check the page after lock_page whether it was truncated or not.

Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>

fs/f2fs/checkpoint.c

@@ -65,6 +65,10 @@ repeat:
 		goto repeat;
 
 	lock_page(page);
+	if (page->mapping != mapping) {
+		f2fs_put_page(page, 1);
+		goto repeat;
+	}
 out:
 	mark_page_accessed(page);
 	return page;