compat_ioctl: move cdrom handlers to block/compat_ioctl.c

These are shared by all cd-rom drivers and should have common
handlers. Do slight cosmetic cleanups in the process.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>