block: add a cdrom_device_info pointer to struct gendisk

Add a pointer to the CDROM information structure to struct gendisk.
This will allow various removable media file systems to call directly
into the CDROM layer instead of abusing ioctls with kernel pointers.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Damien Le Moal <damien.lemoal@wdc.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>