kubelet/cm: GetResourceStats -> MemoryUsage

Commit cc50aa9dfb introduced GetResourceStats, a method which collected
all the statistics from various cgroup controllers, only to discard all
of the info collected except a single value (memory usage).

While one may argue that this method can potentially be used from other
places, this did not happen since it was added 4+ years ago.

Let's streamline this code and only collect what we need, i.e. memory
usage. Rename the method accordingly.

While at it, fix pkg/kubelet/cm/cgroup_manager_unsupported.go to not
instantiate a new error every time a method is called.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

pkg/kubelet/cm/qos_container_manager_linux.go

@@ -28,7 +28,7 @@ import (
 
 	units "github.com/docker/go-units"
 	cgroupfs "github.com/opencontainers/runc/libcontainer/cgroups/fs"
-	"k8s.io/api/core/v1"
+	v1 "k8s.io/api/core/v1"
 	utilfeature "k8s.io/apiserver/pkg/util/feature"
 	"k8s.io/kubernetes/pkg/api/v1/resource"
 	v1qos "k8s.io/kubernetes/pkg/apis/core/v1/helper/qos"
@@ -247,12 +247,11 @@ func (m *qosContainerManagerImpl) retrySetMemoryReserve(configs map[v1.PodQOSCla
 	// Attempt to set the limit near the current usage to put pressure
 	// on the cgroup and prevent further growth.
 	for qos, config := range configs {
-		stats, err := m.cgroupManager.GetResourceStats(config.Name)
+		usage, err := m.cgroupManager.MemoryUsage(config.Name)
 		if err != nil {
 			klog.V(2).InfoS("Failed to get resource stats", "err", err)
 			return
 		}
-		usage := stats.MemoryStats.Usage
 
 		// Because there is no good way to determine of the original Update()
 		// on the memory resource was successful, we determine failure of the