RFR: 8287107: CgroupSubsystemFactory.setCgroupV2Path asserts with freezer controller [v3]

[Ioi Lam]

> This PR fixes a bug found on an Ubuntu host that's mostly running with cgroupv2, but there's a controller (freezer) that is mounted in cgroupv1 mode.
> 
> The container support code in the VM and JDK checks if we have simultaneously mounted v1 and v2 containers. If so, we revert to "hybrid" mode (which is essentially v1).
> 
> However, the "hybrid checks" only considers the following controllers that Java cares about:
> 
> - cpu
> - cpuacct
> - cpuset
> - blkio
> - memory
> - pids
> 
> If only "freezer" is mounted in v1 mode, Java will start in V2 mode. Later, when `setCgroupV2Path()` sees the first line in /proc/self/cgroup, it runs into the assert.
> 
> 
> $ cat /proc/self/cgroup
> 1:freezer:/
> 0::/user.slice/user-1001.slice/session-85.scope
> 
> 
> The fix is simple -- when we get to `setCgroupV2Path()`, we have already decided that the system has not mounted any v1 controllers that we care about, so we should just ignore all the v1 controllers (which has a non-empty name).

Ioi Lam has updated the pull request incrementally with one additional commit since the last revision:

  fixed comments

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/8858/files
  - new: https://git.openjdk.java.net/jdk/pull/8858/files/1f17b6e8..9134182e

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=8858&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=8858&range=01-02

  Stats: 8 lines in 1 file changed: 4 ins; 4 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8858.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8858/head:pull/8858

PR: https://git.openjdk.java.net/jdk/pull/8858