/proc/diskstats: Handle cgroup2

[AlexanderS]

Required information

• Distribution:
  • Debian GNU/Linux 11 (bullseye)
  • Linux 5.10.0-28-amd64 #1 SMP Debian 5.10.209-2 (2024-01-31) x86_64 GNU/Linux
• LXCFS version: 4.0.7

Issue description

With cgroup2 the disk io stats are not virtulized. Looking at the code in https://github.com/lxc/lxcfs/blob/main/src/proc_fuse.c#L579 it seems, that only blkio controller of cgroup1 is handled.

cgroup2 seems to provide the required information in /sys/fs/cgroup/lxc.payload.<container name>/io.stat.

[stgraber]

I can't easily tell if there was a good reason that this wasn't done for cgroup2.

But in any case, if it's doable with the information exposed by cgroup2, then we should use that.
If it's not, then LXCFS should render an empty file instead.