target/iblock: fix holder printing in iblock_show_configfs_dev_params

bd_contains is never NULL for an open block device.  In addition ibd_bd
is always set to a block device that was exclusively opened by the
target code, so the holder is guranteed to be ib_dev as well.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>