io_uring: fail links more in io_submit_sqe()

Instead of marking a link with REQ_F_FAIL_LINK on an error and delaying
its failing to the caller, do it eagerly right when after getting an
error in io_submit_sqe(). This renders FAIL_LINK checks in
io_queue_link_head() useless and we can skip it.

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