libct: don't send config to nsexec when joining an existing timens

We should configure the process's timens offset only when we need to
create new time namespace, we shouldn't do it if we are joining an
existing time namespace. (#4635)

Signed-off-by: lifubang <lifubang@acmcoder.com>

libcontainer/container_linux.go

@@ -1146,8 +1146,9 @@ func (c *Container) bootstrapData(cloneFlags uintptr, nsMaps map[configs.Namespa
 		Value: c.config.RootlessEUID,
 	})
 
-	// write boottime and monotonic time ns offsets.
-	if c.config.TimeOffsets != nil {
+	// write boottime and monotonic time ns offsets only when we are not joining an existing time ns
+	_, joinExistingTime := nsMaps[configs.NEWTIME]
+	if !joinExistingTime && c.config.TimeOffsets != nil {
 		var offsetSpec bytes.Buffer
 		for clock, offset := range c.config.TimeOffsets {
 			fmt.Fprintf(&offsetSpec, "%s %d %d\n", clock, offset.Secs, offset.Nanosecs)