Feature: Add oom monitor for cgroupv2.

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Add oom monitor for cgroupv2.

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@kzys @dims @caniszczyk @tianon Hi, cgroup v2 version of oom monitor is pushed, please review, thx!

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#229 Implementation of cgroup v1 oom monitor is here.

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@mxpv @jsturtevant @Burning1020

[mxpv]

Thanks for the contribution!
I left a few comments.
Also clippy on CI suggests a few refactorings.

[mxpv]

You can move this before spawn to handle error properly.
And then move to thread.

[mxpv]

Also possibly you could extend the function to return a stream of inotify_event events to simplify the logic of this loop?

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Hmm, sorry, I don't get you. Are you suggesting that I should move certain parts of this loop into a separate function in order to simplify it? If that's the case, how to make the function returns a stream of inotify_event? Using a channel for that purpose?

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@mxpv or to modify here as:

fn new_read_inotify_file_function(inotify_file) {
         nread = read(inotify_file)
        // verify events here
        if nread >= size_of::<libc::inotify_event>()
        ....
}
pub async fn register_memory_event_v2() {
    // create inotify file
    inotify_file = ...
    tokio::spawn() {
        loop {
            // mv all verifications of events to a new function
            if new_read_inotify_file_function(inotify_file) {
                            parse_kv_file()
                            .......
            } 
        }
    }
}

[mxpv]

nit:

let Some(oom_kill) = mem_map.get("oom_kill") else {
    return;
}

same for below.

[mxpv]

nit: try lastoom_map.get(&key).copied().unwrap_or(0)

[mxpv]

More rusty variation of this:

    let map = file_string
        .lines()
        .filter(|line| line.split_once(" "))
        .map(|(k, v) {
            let val = val.parse::<u32>()?;
            Ok((k.to_string(), val))
        })
        .collect::<Result<HashMap<_, _>>>()?;