don't mount aufs or overlayfs in overlayfs
don't mount aufs or overlayfs in overlayfs

overlayfs can be mounted in overlayfs, but has the same whiteout problem as in
aufs. aufs mounts but has strange behavior.

tsaleh/garden,tangfeixiong/garden,cloudfoundry-incubator/garden-linux,shashidharatd/garden-linux,cloudfoundry-incubator/garden,cloudfoundry-incubator/garden-linux,tangfeixiong/garden,shashidharatd/garden-linux,cloudfoundry-incubator/garden-linux,shashidharatd/garden-linux,savaki/garden,shashidharatd/garden-linux,savaki/garden,tsaleh/garden,cloudfoundry-incubator/garden