nvme: add APIs for stopping/starting admin queue

Add two APIs for stopping and starting admin queue.

Signed-off-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20211014081710.1871747-2-ming.lei@redhat.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>