This patch has a bunch of io scheduler goodies that are, by now, well
tested in -mm and by self and Nick Piggin. In order of interest:

- Use rbtree data structure for sorting of requests. Even with the
  default queue lengths that are fairly short, this cuts a lot of run
  time for io scheduler intensive work loads. If we go to longer queue
  lengths, it very quickly becomes a necessity.

- Add sysfs interface for the tunables. At the same time, finally kill
  the BLKELVGET/BLKELVSET completely. I made these return -ENOTTY in
  2.5.1, but there are left-overs around the kernel. This old interface
  was never any good, it was centered around just one io scheduler.

The io scheduler core itself has received count less hours of tuning by
myself and Nick, should be in pretty good shape. Please apply.

Andrew, I made some sysfs changes to the version from 2.5.56-mm1. It
didn't even compile without warnings (or work, for that matter), as the
sysfs store/show procedures needed updating. Hmm?