pktcdvd: add compat_ioctl handler

pkt_ioctl() implements the generic SCSI_IOCTL_SEND_COMMAND
and some cdrom ioctls by forwarding to the underlying block
device. For compat_ioctl handling, this always takes a
roundtrip through fs/compat_ioctl.c that we should try
to avoid, at least for the compatible commands.

CDROM_SEND_PACKET is an exception here, it requires special
translation in compat_blkdev_driver_ioctl().

CDROM_LAST_WRITTEN has no compat handling at the moment.

Cc: Jens Axboe <axboe@kernel.dk>
Cc: linux-block@vger.kernel.org
Signed-off-by: Arnd Bergmann <arnd@arndb.de>