From: Trond Myklebust <trond.myklebust@fys.uio.no>

The following reversion is what fixes my regression.  That puts the
sequential read numbers back to the 2.6.0 values of ~140MB/sec (from the
current 2.6.1 values of 14MB/second)...

We were triggering I/O of the `ahead' when we hit the last page in the
`current' window.  That's bad because it gives no pipelining at all.

So go back to full pipelining.

It's not at all clear why this change made a 10x difference in NFS
throughput.