nvme-loop: always quiesce and cancel commands before destroying admin q

Once ->init_ctrl_finish() is called there may be commands outstanding,
so we should quiesce the admin queue and cancel all commands prior
to call nvme_loop_destroy_admin_queue().

Signed-off-by: Hannes Reinecke <hare@suse.de>
Tested-by: Mark O'Donovan <shiftee@posteo.net>
Signed-off-by: Keith Busch <kbusch@kernel.org>