null_blk: refactor init and init errors code paths

Simplify the initialization logic of the three block-layers.

- The queue initialization is split into two parts. This allows reuse of
  code when initializing the sq-, bio- and mq-based layers.
- Set submit_queues default value to 0 and always set it at init time.
- Simplify the init error code paths.

Signed-off-by: Matias Bjorling <m@bjorling.me>
Signed-off-by: Jens Axboe <axboe@kernel.dk>