If there's only one usable, non-optimized path, nvme_round_robin_path()
returns NULL, which is wrong. Fix it by falling back to "old", like in
the single optimized path case. Also, if the active path isn't changed,
there's no need to re-assign the pointer.

Fixes: 3f6e3246 ("nvme-multipath: fix logic for non-optimized paths")
Signed-off-by: NMartin Wilck <mwilck@suse.com>
Signed-off-by: NMartin George <marting@netapp.com>
Reviewed-by: NKeith Busch <kbusch@kernel.org>
Signed-off-by: NSagi Grimberg <sagi@grimberg.me>
Signed-off-by: NJens Axboe <axboe@kernel.dk>