net/http: HTTP/2 Receive Buffer Limit Bypass via Double Flow Control Refund

Go’s HTTP/2 server refunded connection-level flow control twice for the same data: once when the client reset a stream (RST_STREAM) that still had unread data buffered, and again when the handler later read that buffered data. A client that repeatedly sends data and resets streams can therefore make the server buffer more than MaxReceiveBufferPerConnection allows, consuming server memory beyond the configured limit. The total stays bounded by the concurrent stream limit and the per-stream windows. The fix defers the refund until the handler finishes.