f2fs: report error of f2fs_fill_dentries

Report error of f2fs_fill_dentries to ->iterate_shared, otherwise when
error ocurrs, user may just list part of dirents in target directory
without any hints.

Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>