cgroup v2 root stats should not be calculated as v1

Description

From the comments in https://github.com/opencontainers/runc/blob/main/libcontainer/cgroups/fs2/memory.go#L218, it appears the root stats of cgroup v2 was collected in a way to be similar to how v1 is implemented. My understanding, root stats v2 should be collected as memory.current in non-root cgroups, which is supposed to be total memory as opposed to just anon + file.

This issue causes missing a visible amount memory not counted in Usage (e.g. kernel memory) and is misleading. I propose using the value from /proc/meminfo to get the total memory usage for root stat.

Steps to reproduce the issue

1. run a kubelet
2. curl localhost:10255/stats/summary to check the root stats
3. stats got from step 2 do not match the system memory usage

Describe the results you received and expected

root stats' usageBytes should match system memory usage

What version of runc are you using?

runc version 1.1.7
commit: a916309fff0f838eb94e928713dbc3c0d0ac7aa4
spec: 1.0.2-dev
go: go1.20.5
libseccomp: 2.5.4

Host OS information

No response

Host kernel information

No response

Read libcontainer/cgroups/fs2/memory.go around line 218 and compare how root and non-root cgroup v2 memory stats are collected. Check how /proc/meminfo could provide the proposed root value, then reproduce the discrepancy with kubelet and curl localhost:10255/stats/summary. Done means root stats usageBytes match system memory usage without applying the v1 calculation.