io_uring: ensure only sqo_task has file notes

For SQPOLL io_uring we want to have only one file note held by
sqo_task. Add a warning to make sure it holds. It's deep in
io_uring_add_task_file() out of hot path, so shouldn't hurt.
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -9099,6 +9099,10 @@ static int io_uring_add_task_file(struct io_ring_ctx *ctx, struct file *file)
 				fput(file);
 				return ret;
 			}
+
+			/* one and only SQPOLL file note, held by sqo_task */
+			WARN_ON_ONCE((ctx->flags & IORING_SETUP_SQPOLL) &&
+				     current != ctx->sqo_task);
 		}
 		tctx->last = file;
 	}