exclude cgroups slab_reclaimable from container_memory_working_set_bytes

[CharlieR-o-o-t]

Related to issue: #3081

"slab_reclaimable" should be excluded from container_memory_working_set_bytes.

"slab_reclaimable" is only unused cached kernel structured, which can be freed on mem pressure.

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[asbachb]

@cwangVT do you have any opinion on that change?

[fabiand]

@dims it would be great if you could give this PR a look )I think there have been similar ones in the past).

For context see #3081 (comment) , similar #3286

tldr; today, workingset contains reclaimable memory, which then in turn is leading to incorrect decisions.

[CharlieR-o-o-t]

Guys, PR 7 months old. Is anybody able to take a look?

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