RFR: 8287741: Fix of JDK-8287107 (unused cgv1 freezer controller) was incomplete

[Severin Gehwolf]

Please review this hotspot fix follow-up of JDK-8287107. The test which exercised this code was already added with  JDK-8287107. I've now added an assert ensuring that we don't set the cgroup path multiple times. The fix is essentially the same as on the JDK side, skip setting the cgroup path when the hierarchy is non-zero which is equivalent to the empty string match that was done in the Java code.

Testing:
- [x] `test/hotspot/jtreg/containers/cgroup/CgroupSubsystemFactory.java` with fastdebug on x86_64. Asserts before the code fix, passes after.
- [ ] GHA - still running.

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Commit messages:
 - 8287741: Fix of JDK-8287107 was incomplete

Changes: https://git.openjdk.java.net/jdk/pull/9001/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=9001&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8287741
  Stats: 6 lines in 1 file changed: 6 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/9001.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/9001/head:pull/9001

PR: https://git.openjdk.java.net/jdk/pull/9001