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

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

[ Upstream commit 3aefb5ee843fbe4789d03bb181e190d462df95e4 ]

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

Fixes: 41cd8b70c37a ("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>
Signed-off-by: Sasha Levin <sashal@kernel.org>