libct/cgroupv1: privatize [New]NotFoundError

Those two are not used outside of the package and should not be public.

While at it, privatize the Subsystem field of NotFoundError.

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

Author: kolyshkin

libcontainer/cgroups/cgroupv1/utils.go

@@ -25,25 +25,25 @@ var (
 	errUnified = errors.New("not implemented for cgroup v2 unified hierarchy")
 )
 
-type NotFoundError struct {
-	Subsystem string
+type notFoundError struct {
+	subsystem string
 }
 
-func (e *NotFoundError) Error() string {
-	return fmt.Sprintf("mountpoint for %s not found", e.Subsystem)
+func (e *notFoundError) Error() string {
+	return fmt.Sprintf("mountpoint for %s not found", e.subsystem)
 }
 
-func NewNotFoundError(sub string) error {
-	return &NotFoundError{
-		Subsystem: sub,
+func newNotFoundError(sub string) error {
+	return &notFoundError{
+		subsystem: sub,
 	}
 }
 
 func IsNotFound(err error) bool {
 	if err == nil {
 		return false
 	}
-	_, ok := err.(*NotFoundError)
+	_, ok := err.(*notFoundError)
 	return ok
 }
 
@@ -65,7 +65,7 @@ func FindCgroupMountpointAndRoot(cgroupPath, subsystem string) (string, string,
 	// parsing it directly sped up x10 times because of not using Sscanf.
 	// It was one of two major performance drawbacks in container start.
 	if !isSubsystemAvailable(subsystem) {
-		return "", "", NewNotFoundError(subsystem)
+		return "", "", newNotFoundError(subsystem)
 	}
 
 	f, err := os.Open("/proc/self/mountinfo")
@@ -97,7 +97,7 @@ func findCgroupMountpointAndRootFromReader(reader io.Reader, cgroupPath, subsyst
 		return "", "", err
 	}
 
-	return "", "", NewNotFoundError(subsystem)
+	return "", "", newNotFoundError(subsystem)
 }
 
 func isSubsystemAvailable(subsystem string) bool {
@@ -340,5 +340,5 @@ func getControllerPath(subsystem string, cgroups map[string]string) (string, err
 		return p, nil
 	}
 
-	return "", NewNotFoundError(subsystem)
+	return "", newNotFoundError(subsystem)
 }