nvme: tighten up state check for namespace scanning

We only should be scanning namespaces if the controller is live.  Currently
we call the function just before setting it live, so fix the code up to
move the call to nvme_queue_scan to just below the state change.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Acked-by Jon Derrick: <jonathan.derrick@intel.com>
Signed-off-by: Jens Axboe <axboe@fb.com>