overlay: test creating lower hardlinks for copied up files

overlay: test creating lower hardlinks for copied up files

kernel v4.13 introduced the index=on feature for not breaking
hardlinks on copy-up.  With the index feature enabled a regression
was introduced - lower files that are hardlined while overlay is
offline can result in lookup error after overlay in mounted.

The regression was fixed by upstream commit
  6eaf011144af ovl: fix EIO from lookup of non-indexed upper
that was merged to v4.14-rc7.

This test verifies that behavior is sane after creating lower
hardlinks for copied up files while overlayfs is offline.

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