libct/cgroups.OpenFile: clean "file" argument

This prevents potential exploit of using "../" in cgroups.OpenFile
(as well as other methods that use OpenFile) to read or write to
other cgroups.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>
(cherry picked from commit 2c9598c886)
Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

libcontainer/cgroups/file.go

@@ -10,6 +10,7 @@ import (
 	"strings"
 	"sync"
 
+	"github.com/opencontainers/runc/libcontainer/utils"
 	"github.com/sirupsen/logrus"
 	"golang.org/x/sys/unix"
 )
@@ -122,7 +123,7 @@ func openFile(dir, file string, flags int) (*os.File, error) {
 		flags |= os.O_TRUNC | os.O_CREATE
 		mode = 0o600
 	}
-	path := path.Join(dir, file)
+	path := path.Join(dir, utils.CleanPath(file))
 	if prepareOpenat2() != nil {
 		return openFallback(path, flags, mode)
 	}