Title: cpuset effective_cpus/effective_mems use wrong file names for cgroup v2

Description:

The cpuset controller is reading the wrong files for effective CPUs/mems on cgroup v2 systems.

In src/fs/cpuset.rs lines 282 and 288, the code reads:

• cpuset.effective_cpus
• cpuset.effective_mems

However, these file names changed between v1 and v2:

cgroup v1:

• cpuset.effective_cpus
• cpuset.effective_mems

cgroup v2:

• cpuset.cpus.effective
• cpuset.mems.effective

The current implementation always uses the v1 names even though the controller has a v2 flag. This causes cpuset().effective_cpus to return an empty vector on v2 systems since the file doesn't exist.

Reproduction:

On a system using cgroup v2:

let cg = Cgroup::load(Box::new(V2::new()), PathBuf::from("some/cgroup"));
let cpuset_ctrl: &CpuSetController = cg.controller_of().unwrap();
let cpuset = cpuset_ctrl.cpuset();
// cpuset.effective_cpus will be empty even if CPUs are assigned

Fix:

The cpuset() method should check self.v2 and use the appropriate file names:

effective_cpus: {
    let file = if self.v2 {
        "cpuset.cpus.effective"
    } else {
        "cpuset.effective_cpus"
    };
    self.open_path(file, false)
        .and_then(read_string_from)
        .and_then(parse_range)
        .unwrap_or_default()
},

References:

• cgroup v1 cpusets docs
• cgroup v2 docs