overlay: Test mount error cases with overlapping layers

overlay: Test mount error cases with overlapping layers

Add tests for overlayfs mounts with:
- Same upperdir/lowerdir
- Overlapping upperdir/lowerdir
- Overlapping lowerdir layers
- Overlapping lowerdir with other mount upperdir/workdir

Add test for moving layer into another after mount.

Overlapping layers on mount or lookup results in ELOOP.
Overlapping lowerdir with other mount upperdir/workdir
result in EBUSY.

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