NVMe: Memory barrier before queue_count is incremented

Protects against reordering and/or preempting which would allow the
kthread to access the queue descriptor before it is set up

Signed-off-by: Jon Derrick <jonathan.derrick@intel.com>
Acked-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Jens Axboe <axboe@fb.com>