cgroupv2 support

[elelayan]

Hi,
thank you for sharing this nice tool!
I've needed to adapt it to cgroups v2 for my slurm cluster:

• there is no cpu or memory (/sys/fs/cgroup/{cpu,memory}) at the root of the path
• there is no cpuacct.usage, but we can use the usage_usec field in the cpu.stat file
• we have to look in the slurmstepd.scope:
  /sys/fs/cgroup/system.slice/slurmstepd.scope/job_%v instead of /sys/fs/cgroup/.../slurm/uid_%v/job_%v

It is a small patch to cmd/jobperf/nodestats.go. I'll propose a MR shortly.

[dougnd]

Thanks for looking into this! An MR would be appreciated -- there's a good chance we'll be moving to cgroupv2 in the not too distant future.