Support systems using the new cgroup v2 structure

With cgroup v2, the location of the pids.max file changed and so did the /proc/self/cgroup file

new /proc/self/cgroup file
`
0::/user.slice/user-500.slice/session-14.scope
`

old file:
`
11:pids:/user.slice/user-500.slice/session-2.scope
10:blkio:/user.slice
9:net_cls,net_prio:/
8:perf_event:/
...
`

There is no directory per subsystem (e.g. /sys/fs/cgroup/pids) any more, all files are now in one directory.

fixes: #3085

internal/util/pidlimit.go

@@ -27,32 +27,48 @@ import (
 )
 
 const (
-	procCgroup    = "/proc/self/cgroup"
-	sysPidsMaxFmt = "/sys/fs/cgroup/pids%s/pids.max"
+	procCgroup            = "/proc/self/cgroup"
+	sysPidsMaxFmtCgroupV1 = "/sys/fs/cgroup/pids%s/pids.max"
+	sysPidsMaxFmtCgroupV2 = "/sys/fs/cgroup%s/pids.max"
 )
 
 // return the cgouprs "pids.max" file of the current process
 //
-// find the line containing the pids group from the /proc/self/cgroup file
-// $ grep 'pids' /proc/self/cgroup
+// There are two different cgroup structures, cgroup v1 and v2
+//
+// For cgroup v1, find the line containing the pids group from the /proc/self/cgroup file
+// $ grep ':pids:' /proc/self/cgroup
 // 7:pids:/kubepods.slice/kubepods-besteffort.slice/....scope
 // $ cat /sys/fs/cgroup/pids + *.scope + /pids.max.
+//
+// The entry for cgroup v2 is always in the format "0::...scope", no subsystem given.
+// (see https://www.kernel.org/doc/Documentation/cgroup-v2.txt)
+
 func getCgroupPidsFile() (string, error) {
 	cgroup, err := os.Open(procCgroup)
 	if err != nil {
 		return "", err
 	}
 	defer cgroup.Close() // #nosec: error on close is not critical here
 
+	pidsMax := ""
 	scanner := bufio.NewScanner(cgroup)
 	var slice string
 	for scanner.Scan() {
 		parts := strings.SplitN(scanner.Text(), ":", 3)
 		if parts == nil || len(parts) < 3 {
 			continue
 		}
+		// No cgroup subsystem given, then it is cgroupv2
+		if parts[0] == "0" && parts[1] == "" {
+			slice = parts[2]
+			pidsMax = fmt.Sprintf(sysPidsMaxFmtCgroupV2, slice)
+
+			break
+		}
 		if parts[1] == "pids" {
 			slice = parts[2]
+			pidsMax = fmt.Sprintf(sysPidsMaxFmtCgroupV1, slice)
 
 			break
 		}
@@ -61,8 +77,6 @@ func getCgroupPidsFile() (string, error) {
 		return "", fmt.Errorf("could not find a cgroup for 'pids'")
 	}
 
-	pidsMax := fmt.Sprintf(sysPidsMaxFmt, slice)
-
 	return pidsMax, nil
 }