io_uring: move defer_list to slow data

draining is slow path, move defer_list to the end where slow data lives
inside the context.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/e16379391ca72b490afdd24e8944baab849b4a7b.1655310733.git.asml.silence@gmail.com


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