http2: fix CloseNotify data race

Fixes golang/go#17868

Change-Id: I7f9d85f404198d68757d7ca5bb8b1e7aafaafa8b
Reviewed-on: https://go-review.googlesource.com/33013
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Tom Bergan <tombergan@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

server.go

@@ -2276,8 +2276,9 @@ func (w *responseWriter) CloseNotify() <-chan bool {
 	if ch == nil {
 		ch = make(chan bool, 1)
 		rws.closeNotifierCh = ch
+		cw := rws.stream.cw
 		go func() {
-			rws.stream.cw.Wait() // wait for close
+			cw.Wait() // wait for close
 			ch <- true
 		}()
 	}