iotests/081: Test rewrite-corrupted without WRITE

Test what happens when a rewrite-corrupted quorum node performs such a
rewrite, while there is no parent that has taken the WRITE permission.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20201113211718.261671-4-mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>