block: no need to initialize rq->cmd in prepare_flush_fn hook
The block layer initializes rq->cmd (queue_flush calls rq_init) so
prepare_flush_fn hooks don't need to do that.
The purpose of this patch is to remove sizeof(rq->cmd), as a
preparation for large command support, which changes rq->cmd from the
static array to a pointer. sizeof(rq->cmd) will not make sense.