block: add a queue_has_disk helper

Add a helper to check if a gendisk is associated with a request_queue.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Link: https://lore.kernel.org/r/20210809141744.1203023-4-hch@lst.de


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