MaskPaths: support directory

For example, the /sys/firmware directory should be masked because it can contain some sensitive files:
  - /sys/firmware/acpi/tables/{SLIC,MSDM}: Windows license information:
  - /sys/firmware/ibft/target0/chap-secret: iSCSI CHAP secret

Signed-off-by: Akihiro Suda <suda.akihiro@lab.ntt.co.jp>

libcontainer/standard_init_linux.go

@@ -108,7 +108,7 @@ func (l *linuxStandardInit) Init() error {
 		}
 	}
 	for _, path := range l.config.Config.MaskPaths {
-		if err := maskFile(path); err != nil {
+		if err := maskPath(path); err != nil {
 			return err
 		}
 	}