libct/cgroups/utils: make FindCgroupMountpoint* v1-specific

It's bad and wrong to use these functions for any cgroupv2 code,
and there are no existing users (in runc, at least).

Make them return an error in such case.

Also, remove the cgroupv2-specific handling from
findCgroupMountpointAndRootFromReader().

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

libcontainer/cgroups/utils.go

@@ -54,13 +54,17 @@ func IsCgroup2UnifiedMode() bool {
 // https://www.kernel.org/doc/Documentation/cgroup-v1/cgroups.txt
 func FindCgroupMountpoint(cgroupPath, subsystem string) (string, error) {
 	if IsCgroup2UnifiedMode() {
-		return unifiedMountpoint, nil
+		return "", errUnified
 	}
 	mnt, _, err := FindCgroupMountpointAndRoot(cgroupPath, subsystem)
 	return mnt, err
 }
 
 func FindCgroupMountpointAndRoot(cgroupPath, subsystem string) (string, string, error) {
+	if IsCgroup2UnifiedMode() {
+		return "", "", errUnified
+	}
+
 	// We are not using mount.GetMounts() because it's super-inefficient,
 	// parsing it directly sped up x10 times because of not using Sscanf.
 	// It was one of two major performance drawbacks in container start.
@@ -74,10 +78,6 @@ func FindCgroupMountpointAndRoot(cgroupPath, subsystem string) (string, string,
 	}
 	defer f.Close()
 
-	if IsCgroup2UnifiedMode() {
-		subsystem = ""
-	}
-
 	return findCgroupMountpointAndRootFromReader(f, cgroupPath, subsystem)
 }
 
@@ -91,7 +91,7 @@ func findCgroupMountpointAndRootFromReader(reader io.Reader, cgroupPath, subsyst
 		}
 		if strings.HasPrefix(fields[4], cgroupPath) {
 			for _, opt := range strings.Split(fields[len(fields)-1], ",") {
-				if (subsystem == "" && fields[9] == "cgroup2") || opt == subsystem {
+				if opt == subsystem {
 					return fields[4], fields[3], nil
 				}
 			}