block: add completion handler for fast path

The batched completions only deal with non-partial requests anyway,
and it doesn't deal with any requests that have errors. Add a completion
handler that assumes it's a full request and that it's all being ended
successfully.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>