block: queue flag cleanup

We have QUEUE_FLAG_DEFAULT defined, but it's not used anymore since
the legacy IO stack is gone. Kill it.

Sanitize the queue flags in general, they use spaces (for some
reason), and the space is pretty sparse. With the flags renumbered,
we can more clearly see how many we have available.

Signed-off-by: Jens Axboe <axboe@kernel.dk>