stop using ioctl_by_bdev for file system access to CDROMs v2

[Christoph Hellwig]

Hi Jens,

except for the DASD case under discussion the last users of ioctl_by_bdev
are the file system drivers that want to query CDROM information using
ioctls.  This series switches them to use function calls directly into
the CDROM midlayer instead, which implies:

 - adding a cdrom_device_info pointer to the gendisk, so that file systems
   can find it without going to the low-level driver first
 - ensuring that the CDROM midlayer (which isn't a lot of code) is built
   in if the file systems are built in so that they can actually call the
   exported functions

Changes since v1:
 - fix up the no-CDROM error case in isofs_get_last_session to return 0
   instead of -EINVAL.