[PATCHSET v4] sbitmap optimizations

[Jens Axboe <axboe@kernel.dk>]

This versions tests out solid, and we're still seeing the same
improvements.

Changes:

- Lock map index for the move. This eliminates the race completely,
  since it's now not possible to find ->cleared == 0 while swap of
  bits is in progress. The previous version was fine for users that
  re-check after having added themselves to the waitqueue, but we
  have users that don't re-check after getting failure. This works
  for both.

- Add the new states to the blk-mq debugfs output.

- Wrap the waitqueue in a sbitmap waitqueue, so we can ensure that
  we account it properly. This means that any kind of prep+finish
  on the waitqueue will work fine, just like before.

-- 
Jens Axboe