libcontainer: simplify createCgroupsv2Path

fmt.Sprintf is slow and is not needed here, string concatenation would
be sufficient. It is also redundant to convert []byte from string and
back, since `bytes` package now provides the same functions as `strings`.

Use Fields() instead of TrimSpace() and Split(), mainly for readability
(note Fields() is somewhat slower than Split() but here it doesn't
matter much).

Use Join() to prepend the plus signs.

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

libcontainer/cgroups/systemd/unified_hierarchy.go

@@ -3,6 +3,7 @@
 package systemd
 
 import (
+	"bytes"
 	"fmt"
 	"io/ioutil"
 	"math"
@@ -211,15 +212,8 @@ func createCgroupsv2Path(path string) (Err error) {
 		return fmt.Errorf("invalid cgroup path %s", path)
 	}
 
-	res := ""
-	for i, c := range strings.Split(strings.TrimSpace(string(content)), " ") {
-		if i == 0 {
-			res = fmt.Sprintf("+%s", c)
-		} else {
-			res = res + fmt.Sprintf(" +%s", c)
-		}
-	}
-	resByte := []byte(res)
+	ctrs := bytes.Fields(content)
+	res := append([]byte("+"), bytes.Join(ctrs, []byte(" +"))...)
 
 	current := "/sys/fs"
 	elements := strings.Split(path, "/")
@@ -240,7 +234,7 @@ func createCgroupsv2Path(path string) (Err error) {
 			}
 		}
 		if i < len(elements[3:])-1 {
-			if err := ioutil.WriteFile(filepath.Join(current, "cgroup.subtree_control"), resByte, 0755); err != nil {
+			if err := ioutil.WriteFile(filepath.Join(current, "cgroup.subtree_control"), res, 0755); err != nil {
 				return err
 			}
 		}