go back to 256 requests per queue

The request queue was increased from 256 slots to 512 in 2.5.20. The throughput of `dbench 128' on Randy's 384 megabyte machine fell 40%.

We do need to understand why that happened, and what we can learn from it. But in the meanwhile I'd suggest that we go back to 256 slots so that this known problem doesn't impact people's evaluation and tuning of 2.5 performance.

@@ -2002,8 +2002,8 @@ int __init blk_dev_init(void)
 	queue_nr_requests = (total_ram >> 8) & ~15;	/* One per quarter-megabyte */
 	if (queue_nr_requests < 32)
 		queue_nr_requests = 32;
-	if (queue_nr_requests > 512)
-		queue_nr_requests = 512;
+	if (queue_nr_requests > 256)
+		queue_nr_requests = 256;
 
 	/*
 	 * Batch frees according to queue length

-