Updated direct IO optimization patchkit v2

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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