overlay/078: Fix cleanup with unpatched kernel

overlay/078: Fix cleanup with unpatched kernel

Darrick wrote: "...
I noticed a massive regression with this week's fstests,
probably because something didn't get cleaned up properly:
...
+rm: cannot remove '/opt/ovl-upper/testfile': Operation not permitted
...

and then the tests after it (e.g. generic/030) fail with:

+mount: /opt/ovl-mnt: mount(2) system call failed: Stale file handle.
..."

Link: https://lore.kernel.org/fstests/20210802230727.GC3601425@magnolia/
Reported-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>