io_uring: drain next sqe instead of shadowing

There's an issue with the shadow drain logic in that we drop the
completion lock after deciding to defer a request, then re-grab it later
and assume that the state is still the same. In the mean time, someone
else completing a request could have found and issued it. This can cause
a stall in the queue, by having a shadow request inserted that nobody is
going to drain.

Additionally, if we fail allocating the shadow request, we simply ignore
the drain.

Instead of using a shadow request, defer the next request/link instead.
This also has the following advantages:

- removes semi-duplicated code
- doesn't allocate memory for shadows
- works better if only the head marked for drain
- doesn't need complex synchronisation

On the flip side, it removes the shadow->seq ==
last_drain_in_in_link->seq optimization. That shouldn't be a common
case, and can always be added back, if needed.

Fixes: 4fe2c963 ("io_uring: add support for link with drain")
Cc: Jackie Liu <liuyun01@kylinos.cn>
Reported-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>