nvme: switch to RCU freeing the namespace

nvme: switch to RCU freeing the namespace

Switch to RCU freeing the namespace structure so that
nvme_start_queues, nvme_stop_queues and nvme_kill_queues would
be able to get away with only a RCU read side critical section.

Suggested-by: Christoph Hellwig <[email protected]>
Signed-off-by: Ming Lin <[email protected]>
Reviewed-by: Christoph Hellwig <[email protected]>
Reviewed-by: Keith Busch <[email protected]>
Reviewed-by: Sagi Grimerg <[email protected]>
Signed-off-by: Jens Axboe <[email protected]>