overlay: test unstable inode number

overlay: test unstable inode number

Introduce a new test to demonstrate a known issue with overlayfs:
- stat file A shows inode number X
- modify A to trigger copy up
- stat file A shows inode number Y != X

Also test if d_ino of readdir entries changes after copy up.

There is a POC patch by Miklos Szeredi that fixes this issue.

Not dealing with hardlinks here, because there is more to test
then stable inode number.
Hardlinks will get a test of their own.

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