pktcdvd: fix BUG caused by sysfs module reference semantics change

pkt_setup_dev() expects module reference to be held on invocation.
This used to be true for sysfs callbacks but not anymore.  Test and
grab module reference around pkt_setup_dev() in
class_pktcdvd_store_add().

Signed-off-by: Tejun Heo <htejun@gmail.com>
Acked-by: Peter Osterlund <petero2@telia.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>