block: refine the disk_live check in del_gendisk

hidden gendisks will never be marked live.

Fixes: 40b3a52f ("block: add a sanity check for a live disk in del_gendisk")
Reported-by: NBruno Goncalves <bgoncalv@redhat.com>
Signed-off-by: NChristoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20210824144310.1487816-1-hch@lst.deSigned-off-by: NJens Axboe <axboe@kernel.dk>

block/genhd.c

@@ -556,7 +556,7 @@ void del_gendisk(struct gendisk *disk)
 {
 	might_sleep();
 
-	if (WARN_ON_ONCE(!disk_live(disk)))
+	if (WARN_ON_ONCE(!disk_live(disk) && !(disk->flags & GENHD_FL_HIDDEN)))
 		return;
 
 	blk_integrity_del(disk);