nvdimm/btt: do not call del_gendisk() if not needed

del_gendisk() should not called if the disk has not been added. Fix this.

Fixes: 41cd8b70 ("libnvdimm, btt: add support for blk integrity")
Reviewed-by: Dan Williams <dan.j.williams@intel.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
Link: https://lore.kernel.org/r/20211103165843.1402142-1-mcgrof@kernel.org


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