io/ioutil: fix data race under the race detector

See issue 3970 (it's already marked as Fixed).

R=rsc, minux.ma
CC=golang-dev
https://golang.org/cl/6624059

src/pkg/io/ioutil/blackhole.go

+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build !race
+
+package ioutil
+
+var blackHoleBuf = make([]byte, 8192)
+
+func blackHole() []byte {
+	return blackHoleBuf
+}

src/pkg/io/ioutil/blackhole_race.go

+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build race
+
+package ioutil
+
+// Replaces the normal fast implementation with slower but formally correct one.
+
+func blackHole() []byte {
+	return make([]byte, 8192)
+}

src/pkg/io/ioutil/ioutil.go

@@ -130,12 +130,11 @@ func (devNull) Write(p []byte) (int, error) {
 	return len(p), nil
 }
 
-var blackHole = make([]byte, 8192)
-
 func (devNull) ReadFrom(r io.Reader) (n int64, err error) {
+	buf := blackHole()
 	readSize := 0
 	for {
-		readSize, err = r.Read(blackHole)
+		readSize, err = r.Read(buf)
 		n += int64(readSize)
 		if err != nil {
 			if err == io.EOF {