overlay: test encode/decode overlay file handles

overlay: test encode/decode overlay file handles

- Check encode/write/decode/read content of lower/upper file handles
- Check encode/decode/write/read content of lower/upper file handles
- Check decode/read of unlinked lower/upper files and directories
- Check decode/read of lower file handles after copy up, link and unlink

This test requires and enables overlayfs NFS export support. NFS
export support depends on and requires overlayfs index feature.

This test covers only encode/decode of file handles for overlayfs
configuration of all layers on the same base fs.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>