io_uring: fix sequencing issues with linked timeouts

We have an issue with timeout links that are deeper in the submit chain,
because we only handle it upfront, not from later submissions. Move the
prep + issue of the timeout link to the async work prep handler, and do
it normally for non-async queue. If we validate and prepare the timeout
links upfront when we first see them, there's nothing stopping us from
supporting any sort of nesting.

Fixes: 2665abfd

 ("io_uring: add support for linked SQE timeouts")
Reported-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>