[jdk8u-dev] RFR: 8253797: [cgroups v2] Account for the fact that swap accounting is disabled on some systems [v5]

[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.

Jonathan Dowland has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains two commits:

 - 8253797: [cgroups v2] Account for the fact that swap accounting is disabled on some systems
   
   Backport-of: 669361117d30c9904813b6f728714f8d6abe8484
 - 8253939: [TESTBUG] Increase coverage of the cgroups detection code
   
   Reviewed-by: sgehwolf
   Backport-of: 42fc15899115ca61151bbee96b5580d4e4252beb

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Changes: https://git.openjdk.org/jdk8u-dev/pull/185/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk8u-dev&pr=185&range=04
  Stats: 192 lines in 4 files changed: 158 ins; 1 del; 33 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