Allow cgroup creation without attaching a pid

Signed-off-by: Buddha Prakash <buddhap@google.com>

libcontainer/cgroups/fs/cpuset.go

@@ -8,7 +8,6 @@ import (
 	"io/ioutil"
 	"os"
 	"path/filepath"
-	"strconv"
 
 	"github.com/opencontainers/runc/libcontainer/cgroups"
 	"github.com/opencontainers/runc/libcontainer/configs"
@@ -67,7 +66,7 @@ func (s *CpusetGroup) ApplyDir(dir string, cgroup *configs.Cgroup, pid int) erro
 	}
 	// because we are not using d.join we need to place the pid into the procs file
 	// unlike the other subsystems
-	if err := writeFile(dir, "cgroup.procs", strconv.Itoa(pid)); err != nil {
+	if err := cgroups.WriteCgroupProc(dir, pid); err != nil {
 		return err
 	}