io_uring: use completion batching for poll rem/upd

Use __io_req_complete() in io_poll_update(), so we can utilise
completion batching for both update/remove request and the poll
we're killing (if any).

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/e2bdc6c5abd9e9b80f09b86d8823eb1c780362cd.1639605189.git.asml.silence@gmail.com


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