gdrom: Add missing error code

In case of error, 'err' is known to be 0 here, because of the previous
test. Set it to a -ENOMEM instead.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@fb.com>