io_uring: return back links tw run optimisation

io_uring: return back links tw run optimisation

io_submit_flush_completions() may queue new requests for tw execution,
especially true for linked requests. Recheck the tw list for emptiness
after flushing completions.

Note that this doesn't really fix the commit referenced below, but it
does reinstate an optimization that existed before that got merged.

Fixes: f88262e60bb9 ("io_uring: lockless task list")
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/6328acdbb5e60efc762b18003382de077e6e1367.1673887636.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>