overlay/084: Fix test to match new xwhiteouts dir on-disk format

overlay/084: Fix test to match new xwhiteouts dir on-disk format

The xwhiteouts feature, which is tested in this test, was added to
overlayfs in kernel v6.7.

The on-disk format of the xwhiteouts directory was changed in kernel
v6.8-rc2, specfically by commit 420332b94119 ("ovl: mark xwhiteouts
directory with overlay.opaque='x'") and backported to kernel v6.7.3,
so this test now fails on kernel >= v6.8-rc2 and => v6.7.3.

Adapt the test to the new on-disk format and add a hint to make sure
that the on-disk format change is backported to v6.7 based kernels.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Acked-by: Alexander Larsson <alexl@redhat.com>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>