overlay: test mount error cases with exclusive directories

overlay: test mount error cases with exclusive directories

Overlayfs is often used to mount several mounts that share a single
lower dir, but every overlayfs mount should have its own private
upperdir and private workdir.

Overlayfs mount on kernel <= v4.12 does not check if upper/work dirs
are currently in-use by another overlayfs mount on the system and bad
things can happen with such configuration.

Expect EBUSY when trying to mount overlay when:
- Upper dir is in-use by another overlay mount
- Work dir is in-use by another overlay mount

This test does not depend on the overlay index feature.

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