io_uring: keep queue_sqe()'s fail path separately

A preparation path, extracts error path into a separate block. It looks
saner then calling req_set_fail_links() after io_put_req_find_next(), even
though it have been working well.

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