NVMe: queue usage fixes in nvme-scsi

Fixes nvme queue usages in scsi-to-nvme translation code to not get
a queue more often than it is being put, and not use the queue in an
unsafe way without it being locked.

Signed-off-by: Keith Busch <keith.busch@intel.com>
Acked-by: Vishal Verma <vishal.l.verma@linux.intel.com>
Signed-off-by: Matthew Wilcox <matthew.r.wilcox@intel.com>