NVMe: reap completion entries when deleting queue

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>
(cherry picked from commit 604e8c8da8854351496215d269c3fa93859e3fee)

Orabug: 22620486
Signed-off-by: Jason Luo <zhangqing.luo@oracle.com>