io_uring: move *queue_link_head() from common path

Move io_queue_link_head() to links handling code in io_submit_sqe(),
so it wouldn't need extra checks and would have better data locality.

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