NVMe: reap completion entries when deleting queue

Make sure that there are no unprocesssed entries on a completion
queue before deleting it, and check for validity of the CQ
door bell before writing completions to it.

This fixes problems with doing a sysfs reset of the device while
it's handling IO.

Tested-by: Jon Derrick <jonathan.derrick@intel.com>

Signed-off-by: Jens Axboe <axboe@fb.com>