pktcdvd: convert to use attribute groups

pktcdvd: convert to use attribute groups

There is no need to create kobject children of the pktcdvd device just
to display a subdirectory name.  Instead, use a named attribute group
which removes the extra kobjects and also fixes the userspace race where
the device is created yet tools like libudev can not see the attributes
as they think the subdirectories are some other sort of device.

Cc: linux-block@vger.kernel.org
Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://lore.kernel.org/r/20220103162408.742003-1-gregkh@linuxfoundation.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>