nvme-multipath: fix logic for non-optimized paths

Handle the special case where we have exactly one optimized path,
which we should keep using in this case.

Fixes: 75c10e73 ("nvme-multipath: round-robin I/O policy")
Signed off-by: Martin Wilck <mwilck@suse.com>
Signed-off-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Christoph Hellwig <hch@lst.de>