dasd: refactor dasd_ioctl_information

Prepare for in-kernel callers of this functionality.

Signed-off-by: Christoph Hellwig <hch@lst.de>
[sth@de.ibm.com: remove leftover kfree]
Signed-off-by: Stefan Haberland <sth@linux.ibm.com>
Reviewed-by: Peter Oberparleiter <oberpar@linux.ibm.com>
Reviewed-by: Jan Hoeppner <hoeppner@linux.ibm.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>