io_uring: don't submit link on error

If we get an error in io_init_req() for a request that would have been
linked, we break the submission but still issue a partially composed
link, that's nasty, fail it instead.

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