io_uring: run next sqe inline if possible

One major use case of linked commands is the ability to run the next
link inline, if at all possible. This is done correctly for async
offload, but somewhere along the line we lost the ability to do so when
we were able to complete a request without having to punt it. Ensure
that we do so correctly.

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