[jdk8u-dev] RFR: 8287073: NPE from CgroupV2Subsystem.getInstance()

[Severin Gehwolf]

Simple low-risk backport avoiding a potential NPE on some cgroups v2 systems. Patch applies clean after path unshuffeling.

Testing:
 - [x] Container tests on Linux x86_64. Pass.
 - [x] Regression test from #323

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Commit messages:
 - Backport 744b822ab194a0f7ef4e7a4053be32c6a0889efc

Changes: https://git.openjdk.org/jdk8u-dev/pull/324/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk8u-dev&pr=324&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8287073
  Stats: 4 lines in 1 file changed: 1 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk8u-dev/pull/324.diff
  Fetch: git fetch https://git.openjdk.org/jdk8u-dev.git pull/324/head:pull/324

PR: https://git.openjdk.org/jdk8u-dev/pull/324