null_blk: fix softirq completions for queue_mode == 1

Only blk-mq completions have payload attached to the request, for
request_fn mode we have stored it in req->special. This fixes an
oops with queue_mode=1 and softirq completions.
Signed-off-by: Jens Axboe <axboe@fb.com>

drivers/block/null_blk.c

@@ -227,7 +227,10 @@ static void null_cmd_end_timer(struct nullb_cmd *cmd)
 
 static void null_softirq_done_fn(struct request *rq)
 {
-	end_cmd(blk_mq_rq_to_pdu(rq));
+	if (queue_mode == NULL_Q_MQ)
+		end_cmd(blk_mq_rq_to_pdu(rq));
+	else
+		end_cmd(rq->special);
 }
 
 static inline void null_handle_cmd(struct nullb_cmd *cmd)