blk-stat: convert blk-stat bucket callback to signed

In order to allow for filtering of IO based on some other properties
of the request than direction we allow the bucket function to return
an int.

If the bucket callback returns a negative do no count it in the stats
accumulation.

Signed-off-by: Stephen Bates <sbates@raithlin.com>

Fixed up Kyber scheduler stat callback.

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