generic/604: do not run with overlayfs

generic/604: do not run with overlayfs

Overlayfs does not allow mounting over again with the same layers
until umount is fully completed, so is not appropriate for this test
which tries to mount in parallel to umount.

This is manifested as the test failure below when overlayfs strict mount
checks are enabled by enabling the index feature:

$ echo Y > /sys/module/overlay/parameters/index
...
    +mount: /vdf/ovl-mnt: /vdf already mounted or mount point busy.
    +       dmesg(1) may have more information after failed mount system call.
    +mount /vdf /vdf/ovl-mnt failed

Opt-out of this test with overlayfs and remove the hacks that were placed
by commit 06cee932 ("generic/604: Fix for overlayfs") to make the test pass
with overlayfs in the first place.

Tested-by: André Almeida <andrealmeid@igalia.com>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>