nvme: prepare for pairing quiescing and unquiescing

Add two helpers so that we can prepare for pairing quiescing and
unquiescing which will be done in next patch.

Signed-off-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20211014081710.1871747-4-ming.lei@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>