cdrom: fix improper type cast, which can leat to information leak.

There is another cast from unsigned long to int which causes
a bounds check to fail with specially crafted input. The value is
then used as an index in the slot array in cdrom_slot_status().

This issue is similar to CVE-2018-16658 and CVE-2018-10940.

Signed-off-by: Young_X <YangX92@hotmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
static int cdrom_ioctl_select_disc(struct cdrom_device_info *cdi,
		unsigned long arg)
{
	cd_dbg(CD_DO_IOCTL, "entering CDROM_SELECT_DISC\n");

	if (!CDROM_CAN(CDC_SELECT_DISC))
 		return -ENOSYS;
 
 	if (arg != CDSL_CURRENT && arg != CDSL_NONE) {
		if (arg >= cdi->capacity)
 			return -EINVAL;
 	}
 
	/*
	 * ->select_disc is a hook to allow a driver-specific way of
	 * seleting disc.  However, since there is no equivalent hook for
	 * cdrom_slot_status this may not actually be useful...
	 */
	if (cdi->ops->select_disc)
		return cdi->ops->select_disc(cdi, arg);

	cd_dbg(CD_CHANGER, "Using generic cdrom_select_disc()\n");
	return cdrom_select_disc(cdi, arg);
}

static int cdrom_ioctl_select_disc(struct cdrom_device_info *cdi,
		unsigned long arg)
{
	cd_dbg(CD_DO_IOCTL, "entering CDROM_SELECT_DISC\n");

	if (!CDROM_CAN(CDC_SELECT_DISC))
 		return -ENOSYS;
 
 	if (arg != CDSL_CURRENT && arg != CDSL_NONE) {
		if ((int)arg >= cdi->capacity)
 			return -EINVAL;
 	}
 
	/*
	 * ->select_disc is a hook to allow a driver-specific way of
	 * seleting disc.  However, since there is no equivalent hook for
	 * cdrom_slot_status this may not actually be useful...
	 */
	if (cdi->ops->select_disc)
		return cdi->ops->select_disc(cdi, arg);

	cd_dbg(CD_CHANGER, "Using generic cdrom_select_disc()\n");
	return cdrom_select_disc(cdi, arg);
}
