A bunch of fixes

A bunch of fixes

Really should have been split up, but...

- Check this_io_bytes at the bottom of do_io() so that async engines
  have a chance to queue pending IO before deeming this job done.

- dprint() should use log_info(), may not be stdout we want.

- last block bug in get_next_free_block(), if max blocks wasn't a
  multiple of the bitmap size.

- Add more debug points.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>