as-iosched: fix write batch start point

New write batches currently start from where the last one completed.
We have no idea where the head is after switching batches, so this
makes little sense.  Instead, start the next batch from the request
with the earliest deadline in the hope that we avoid a deadline
expiry later on.
Signed-off-by: NAaron Carroll <aaronc@gelato.unsw.edu.au>
Acked-by: NNick Piggin <npiggin@suse.de>
Signed-off-by: NJens Axboe <jens.axboe@oracle.com>

block/as-iosched.c

@@ -1097,7 +1097,8 @@ static int as_dispatch_request(struct request_queue *q, int force)
 		ad->batch_data_dir = REQ_ASYNC;
 		ad->current_write_count = ad->write_batch_count;
 		ad->write_batch_idled = 0;
-		rq = ad->next_rq[ad->batch_data_dir];
+		rq = rq_entry_fifo(ad->fifo_list[REQ_ASYNC].next);
+		ad->last_check_fifo[REQ_ASYNC] = jiffies;
 		goto dispatch_request;
 	}