net/http: fix data race in benchmark

Fixes #7006

R=golang-codereviews, iant
CC=golang-codereviews
https://golang.org/cl/44940044

src/pkg/net/http/serve_test.go

@@ -2436,7 +2436,7 @@ Host: golang.org
 	for i := 0; i < b.N; i++ {
 		conn.Reader = bytes.NewReader(req)
 		ln.conn = conn
-		go Serve(ln, h)
+		Serve(ln, h)
 		<-conn.closec
 	}
 }