overlay: test whiteout check in impure dir

overlay: test whiteout check in impure dir

The unmerged and impure upper directories may contain invalid
whiteouts when we umount && modify lowerdir(e.g. clean up dir) and
remount overlay. This may lead to whiteouts exposure and rmdir
failure.

This case test impure dir's whiteouts check in ovl_iterate() and
ovl_remove_xxx().

Signed-off-by: zhangyi (F) <yi.zhang@huawei.com>
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>