NVMe: Reference count admin queue usage

Since there is no gendisk associated with the admin queue, the driver
needs to hold a reference to it until all open references to the
controller are closed.

This also combines queue cleanup with freeing the tag set since these
should not be separate.

Signed-off-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Jens Axboe <axboe@fb.com>