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: Bruno Goncalves <bgoncalv@redhat.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20210824144310.1487816-1-hch@lst.de


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