f2fs: don't drop dentry pages after fs shutdown

As description in commit "f2fs: don't drop any page on f2fs_cp_error()
case":

"We still provide readdir() after shtudown, so we should keep pages to
avoid additional IOs."

In order to provider lastest directory structure, let's keep dentry
pages in cache after fs shutdown.
Signed-off-by: NChao Yu <yuchao0@huawei.com>
Signed-off-by: NJaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/data.c

@@ -1801,6 +1801,12 @@ static int __write_data_page(struct page *page, bool *submitted,
 	/* we should bypass data pages to proceed the kworkder jobs */
 	if (unlikely(f2fs_cp_error(sbi))) {
 		mapping_set_error(page->mapping, -EIO);
+		/*
+		 * don't drop any dirty dentry pages for keeping lastest
+		 * directory structure.
+		 */
+		if (S_ISDIR(inode->i_mode))
+			goto redirty_out;
 		goto out;
 	}