bcache: move the del_gendisk call out of bcache_device_free

Let the callers call del_gendisk so that we can check if add_disk
has been called properly for the cached device case instead of relying
on the block layer internal GENHD_FL_UP flag.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Coly Li <colyli@suse.de>
Link: https://lore.kernel.org/r/20210809064028.1198327-8-hch@lst.de


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