overlay: fix regression in _repair_overlay_scratch_fs

overlay: fix regression in _repair_overlay_scratch_fs

_repair_overlay_scratch_fs assumed that the base fs is mounted.
This was a wrong assumption to make, and that was exposed by commit
4c6bc456 ("fstests: clean up mount and unmount operations") that
converted open coded umount in generic/332 to _scratch_unmount.

After this change, there errors were observed when running generic/332
if fsck.overlay is installed:

     Check for damage
    +fsck.overlay:[Error]: Faile to resolve upperdir:/vdf/ovl-upper:
                           No such file or directory
    +fsck.overlay failed, err=8
    +umount: /vdf: not mounted.

Fix this by making sure that base fs is mounted before running the
layers check and fix test generic/330 to conform with the umount
conversion patch.

Fixes: 4c6bc456 ("fstests: clean up mount and unmount operations")
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>