dmflakey: fix environment-destroying mistakes in _cleanup_flakey

dmflakey: fix environment-destroying mistakes in _cleanup_flakey

_cleanup_flakey can be called even when _init_flakey hasn't been called.
For example, _require_flakey_with_error_writes does this.

Unfortunately, the patch to add support for external logs and rt devices
will "reset" the SCRATCH_LOGDEV/SCRATCH_RTDEV variables without noticing
that _init_flakey hasn't been called yet.  When this happens, those two
variables will be set to the empty string, with the result that the rest
of the test doesn't use those devices.

To fix this, only reset SCRATCH_LOGDEV/RTDEV to the "NON_FLAKEY" value
if we actually set one.

Fixes: 9c1f3149 ("dmflakey: support external log and realtime devices")
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>