ovl: do not set overlay.opaque for new directories

Enable optimizations only if user opted-in for any of extended features.
If optimization is enabled, it breaks existing use case when a lower layer
directory appears after directory was created on a merged layer. If
overlay.opaque is applied, new files on lower layer are not visible.

Consider the following scenario:
- /lower and /upper are mounted to /merged
- directory /merged/new-dir is created with a file test1
- overlay is unmounted
- directory /lower/new-dir is created with a file test2
- overlay is mounted again

If opaque is applied by default, file test2 is not going to be visible
without explicitly clearing the overlay.opaque attribute

Signed-off-by: Vyacheslav Yurkov <Vyacheslav.Yurkov@bruker.com>
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>