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

[Severin Gehwolf]

This has been implemented for cgroups v1 with [JDK-8250984](https://bugs.openjdk.java.net/browse/JDK-8250984) but was lacking some tooling support for cgroups v2. With podman 2.2.0 release this could now be implemented (and tested). The idea is the same as for the cgroups v1 fix. If we've got no swap limit capabilities, return the memory limit only.

Note that for cgroups v2 doesn't implement CgroupV1Metrics (obviously) and, thus, doesn't have `getMemoryAndSwapFailCount()` and `getMemoryAndSwapMaxUsage()`.

Testing:
- [x] submit testing
- [x] container tests on cgroups v2 with swapaccount=0.
- [x] Manual container tests involving `-XshowSettings:system` on cgroups v2.

Thoughts?

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

Changes: https://git.openjdk.java.net/jdk/pull/1672/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1672&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8253797
  Stats: 45 lines in 2 files changed: 24 ins; 1 del; 20 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1672.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1672/head:pull/1672

PR: https://git.openjdk.java.net/jdk/pull/1672