io_uring: handle completions in the core

Normally request handlers complete requests themselves, if they don't
return an error. For the latter case, the core will complete it for
them.

This is unhandy for pushing opcode handlers further out, as we don't
want a bunch of inline completion code and we don't want to make the
completion path slower than it is now.

Let the core handle any completion, unless the handler explicitly
asks us not to.

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