Fix cache reporting with cgroup hierarchy

[vboginskey]

This makes the behavior of container_memory_cache consistent with container_memory_rss for hierarchical cgroups. See #2008 for more detail.

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[vboginskey]

I signed the CLA.

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CLAs look good, thanks!

[dashpole]

/ok-to-test

[dashpole]

lgtm, thanks!