rbd: kill img_request kref

The reference counter is never increased, so we can as well call
rbd_img_request_destroy() directly and drop the kref.

Signed-off-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>