aufs.Cleanup: optimize

Do not use filepath.Walk() as there's no requirement to recursively
go into every directory under mnt -- a (non-recursive) list of
directories in mnt is sufficient.

With filepath.Walk(), in case some container will fail to unmount,
it'll go through the whole container filesystem which is both
excessive and useless.

This is similar to commit f1a4592297 ("devmapper.shutdown:
optimize")

While at it, raise the priority of "unmount error" message from debug
to a warning. Note we don't have to explicitly add `m` as unmount error (from
pkg/mount) will have it.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>
(cherry picked from commit 8fda12c607)

daemon/graphdriver/aufs/aufs.go

@@ -573,23 +573,20 @@ func (a *Driver) mounted(mountpoint string) (bool, error) {
 
 // Cleanup aufs and unmount all mountpoints
 func (a *Driver) Cleanup() error {
-	var dirs []string
-	if err := filepath.Walk(a.mntPath(), func(path string, info os.FileInfo, err error) error {
-		if err != nil {
-			return err
-		}
-		if !info.IsDir() {
-			return nil
-		}
-		dirs = append(dirs, path)
-		return nil
-	}); err != nil {
-		return err
+	dir := a.mntPath()
+	files, err := ioutil.ReadDir(dir)
+	if err != nil {
+		return errors.Wrap(err, "aufs readdir error")
 	}
+	for _, f := range files {
+		if !f.IsDir() {
+			continue
+		}
+
+		m := path.Join(dir, f.Name())
 
-	for _, m := range dirs {
 		if err := a.unmount(m); err != nil {
-			logger.Debugf("error unmounting %s: %s", m, err)
+			logger.WithError(err).WithField("method", "Cleanup()").Warn()
 		}
 	}
 	return mount.RecursiveUnmount(a.root)