RFR: 8287073: NPE from CgroupV2Subsystem.getInstance()

[Maxim Kartashev]

Following the logic from the comment directly above the changed line, since it doesn't matter which controller we pick, pick any available controller instead of the one called "memory" specifically. This way we are guarded against getting `null` as `anyController`, which is being immediately passed down to `CgroupV2Subsystem.getInstance()` that is unprepared to accept `null` values. 

It is also worth noting that the previous checks (such as that at line 89) make sure that there exist at least one controller in the map.

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Commit messages:
 - 8287073: NPE from CgroupV2Subsystem.getInstance()

Changes: https://git.openjdk.java.net/jdk/pull/8803/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8803&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8287073
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8803.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8803/head:pull/8803

PR: https://git.openjdk.java.net/jdk/pull/8803