overlay: Test constant d_ino feature

overlay: Test constant d_ino feature

This commit adds a test to verify constant d_ino feature. The
following scenarios are checked,
- Parent's (i.e. "..") d_ino must always be calculated because a
  pure dir can be residing inside a merged dir.
- d_ino for "." must always be calculated because the present
  directory can have a copy-up origin.
- Verify d_ino of '.' and '..' before and after dir becomes impure.
  While at it also verify if trusted.overlay.impure xattr is
  set/reset appropriately and invalidation of readdir cache.
- Verify copied up file's (inside a impure dir) d_ino.
- Verify d_ino values corresponding to "." and ".." entries of a
  pure lower dir.
- Verify d_ino of ".." entry of a merged dir.
- Verify pure lower residing in dir which has another lower layer

Signed-off-by: Chandan Rajendra <chandan@linux.vnet.ibm.com>
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>