io_uring: prolong tctx_task_work() with flushing

io_submit_flush_completions() may enqueue linked requests for task_work
execution, so don't leave tctx_task_work() right after the tw list is
exhausted, but try to flush and then retry.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/0755d4c2c36301447c63bdd4146c10477cea4249.1630539342.git.asml.silence@gmail.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>