[jdk8u-dev] RFR: 8253797: [cgroups v2] Account for the fact that swap accounting is disabled on some systems

[Jonathan Dowland]

This is a clean backport of 8253797 (via 11u) for jdk8u-dev for cgroups v2 support.

The changed files are exercised by the jtreg test `TestSystemMetrics.java` which passes on my cgroups v2 system.

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Depends on: https://git.openjdk.org/jdk8u-dev/pull/168

Commit messages:
 - 8253797: [cgroups v2] Account for the fact that swap accounting is disabled on some systems

Changes: https://git.openjdk.org/jdk8u-dev/pull/185/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk8u-dev&pr=185&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8253797
  Stats: 45 lines in 2 files changed: 24 ins; 1 del; 20 mod
  Patch: https://git.openjdk.org/jdk8u-dev/pull/185.diff
  Fetch: git fetch https://git.openjdk.org/jdk8u-dev pull/185/head:pull/185

PR: https://git.openjdk.org/jdk8u-dev/pull/185