Fixed TearDown of NFS with root squash.

NFS plugin should not use IsLikelyNotMountPoint(), as it uses lstat() / stat()
to determine if the NFS volume is still mounted - NFS server may use
root_squash and kubelet may not be allowed to do lstat() / stat() there.

It must use slower IsNotMountPoint() instead, including TearDown() function.

pkg/util/mount/mount.go

@@ -19,6 +19,7 @@ limitations under the License.
 package mount
 
 import (
+	"os"
 	"path/filepath"
 )
 
@@ -208,6 +209,12 @@ func IsNotMountPoint(mounter Interface, file string) (bool, error) {
 	// IsLikelyNotMountPoint provides a quick check
 	// to determine whether file IS A mountpoint
 	notMnt, notMntErr := mounter.IsLikelyNotMountPoint(file)
+	if notMntErr != nil && os.IsPermission(notMntErr) {
+		// We were not allowed to do the simple stat() check, e.g. on NFS with
+		// root_squash. Fall back to /proc/mounts check below.
+		notMnt = true
+		notMntErr = nil
+	}
 	if notMntErr != nil {
 		return notMnt, notMntErr
 	}