Reuse cpuacct stats for cpu subsystem

Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@crosbymichael.com> (github: crosbymichael)

pkg/cgroups/fs/cpu.go

@@ -27,5 +27,7 @@ func (s *cpuGroup) Remove(d *data) error {
 }
 
 func (s *cpuGroup) Stats(d *data) (map[string]float64, error) {
-	return nil, ErrNotSupportStat
+	// we can reuse the cpuacct subsystem to get the cpu stats
+	sys := subsystems["cpuacct"]
+	return sys.Stats(d)
 }