partitions/ibm: fix non-DASD devices

Don't error out if the dasd_biodasdinfo symbol is not available.

Cc: stable@vger.kernel.org
Fixes: 26d7e28e ("s390/dasd: remove ioctl_by_bdev calls")
Reported-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Tested-by: Christian Borntraeger <borntraeger@de.ibm.com>
Reviewed-by: Stefan Haberland <sth@linux.ibm.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>