close xtables lock on errors before returning

iptables/iptables.go

@@ -437,6 +437,7 @@ func (ipt *IPTables) runWithOutput(args []string, stdout io.Writer) error {
 		}
 		ul, err := fmu.tryLock()
 		if err != nil {
+			syscall.Close(fmu.fd)
 			return err
 		}
 		defer ul.Unlock()