[PATCH] go back to 256 requests per queue

[PATCH] 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.