io_uring: fix error handling in io_queue_link_head

In case of an error io_submit_sqe() drops a request and continues
without it, even if the request was a part of a link. Not only it
doesn't cancel links, but also may execute wrong sequence of actions.

Stop consuming sqes, and let the user handle errors.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>