net/http: fix data race on countReader.n

Fixes #4220.

R=dvyukov, bradfitz
CC=golang-dev
https://golang.org/cl/6638053

src/pkg/net/http/serve_test.go

@@ -1063,7 +1063,7 @@ type countReader struct {
 
 func (cr countReader) Read(p []byte) (n int, err error) {
 	n, err = cr.r.Read(p)
-	*cr.n += int64(n)
+	atomic.AddInt64(cr.n, int64(n))
 	return
 }
 
@@ -1081,8 +1081,8 @@ func TestRequestBodyLimit(t *testing.T) {
 	}))
 	defer ts.Close()
 
-	nWritten := int64(0)
-	req, _ := NewRequest("POST", ts.URL, io.LimitReader(countReader{neverEnding('a'), &nWritten}, limit*200))
+	nWritten := new(int64)
+	req, _ := NewRequest("POST", ts.URL, io.LimitReader(countReader{neverEnding('a'), nWritten}, limit*200))
 
 	// Send the POST, but don't care it succeeds or not.  The
 	// remote side is going to reply and then close the TCP
@@ -1095,7 +1095,7 @@ func TestRequestBodyLimit(t *testing.T) {
 	// the remote side hung up on us before we wrote too much.
 	_, _ = DefaultClient.Do(req)
 
-	if nWritten > limit*100 {
+	if atomic.LoadInt64(nWritten) > limit*100 {
 		t.Errorf("handler restricted the request body to %d bytes, but client managed to write %d",
 			limit, nWritten)
 	}