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

include/linux/blkdev.h

@@ -664,6 +664,7 @@ extern void blk_clear_pm_only(struct request_queue *q);
 	dma_map_page_attrs(dev, (bv)->bv_page, (bv)->bv_offset, (bv)->bv_len, \
 	(dir), (attrs))
 
+#define queue_has_disk(q)	((q)->kobj.parent != NULL)
 #define queue_to_disk(q)	(dev_to_disk(kobj_to_dev((q)->kobj.parent)))
 
 static inline bool queue_is_mq(struct request_queue *q)