io_uring: use state completion infra for poll reqs

Use io_req_task_complete() for poll request completions, so it can
utilise state completions and save lots of unnecessary locking.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/ced94cb5a728d8e386c640d052fd3da3f5d6891a.1655371007.git.asml.silence@gmail.com
Reviewed-by: Hao Xu <howeyxu@tencent.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: isilence

io_uring/poll.c

@@ -234,12 +234,8 @@ static void io_poll_task_func(struct io_kiocb *req, bool *locked)
 
 	io_poll_remove_entries(req);
 	io_poll_req_delete(req, ctx);
-	spin_lock(&ctx->completion_lock);
-	req->cqe.flags = 0;
-	__io_req_complete_post(req);
-	io_commit_cqring(ctx);
-	spin_unlock(&ctx->completion_lock);
-	io_cqring_ev_posted(ctx);
+	io_req_set_res(req, req->cqe.res, 0);
+	io_req_task_complete(req, locked);
 }
 
 static void io_apoll_task_func(struct io_kiocb *req, bool *locked)