nvme-loop: check if queue is ready in queue_rq

In case the queue is not LIVE (fully functional and connected at the nvmf
level), we cannot allow any commands other than connect to pass through.

Add a new queue state flag NVME_LOOP_Q_LIVE which is set after nvmf connect
and cleared in queue teardown.

Signed-off-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Christoph Hellwig <hch@lst.de>