runtime: fix memory leak in parallel garbage collector

The work buffer management used by the garbage
collector during parallel collections leaks buffers.
This CL tests for and fixes the leak.

R=golang-dev, dvyukov, r
CC=golang-dev
https://golang.org/cl/5254059

src/pkg/runtime/gc_test.go

@@ -0,0 +1,24 @@
+package runtime_test
+
+import (
+	"runtime"
+	"testing"
+)
+
+func TestGcSys(t *testing.T) {
+	for i := 0; i < 1000000; i++ {
+		workthegc()
+	}
+
+	// Should only be using a few MB.
+	runtime.UpdateMemStats()
+	sys := runtime.MemStats.Sys
+	t.Logf("using %d MB", sys>>20)
+	if sys > 10e6 {
+		t.Fatalf("using too much memory: %d MB", sys>>20)
+	}
+}
+
+func workthegc() []byte {
+	return make([]byte, 1029)
+}