overlay: test mount error cases with index=on

overlay: test mount error cases with index=on

- Upper/lower mismatch
- Index/upper mismatch

With index=on, lowerdir and upperdir are verified using a file
handle stored in trusted.overlay.origin xattr in upperdir and
indexdir.

Failure to verify lowerdir/upperdir on mount results in ESTALE.

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