AIO refactoring/performance improvements/cancellation

This is a respin of the AIO patches that were deferred until 3.11, along
with some other stuff I had queued up.

Changes:

 * Took the dynamic allocation stuff out of the percpu refcounting
   patch, which Tejun was wanting. I split the dynamic bits out into
   another patch, which I may resend later.

 * Changed batch completion to use a singly linked list instead of an rb
   tree; it now calls batch_complete_aio() early if it has to look too
   far down the list.

 * Some batch completion performance improvements, to avoid doing nested
   irqsave/restore (which was the source of a performance regression)
   and not free the kiocbs with irqs disabled.

 * There's also some more assorted refactoring/minor performance
   improvements that had been sitting in my tree for awhile but weren't in
   the patch series that was queued up for 3.10

 * And, the last few patches add cancellation for direct IO; these
   patches are still preliminary but they do work and are useful for
   some simple use cases.
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