Updated direct IO optimization patchkit v2

[Andi Kleen]

Various micro optimizations of the block direct IO path.

This addresses all review comments and adds an additional prefetch
optimization that gives another about 0.3% in a large macro benchmark.
Overall the patchkit is worht about 2.2% in the same macro benchmark.

I also ported it to current mainline, resolving some conflicts with
hch's recent changes to direct-io.

-Andi

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