Fix check for thread being a group leader in memcgroup

The check t->pid == t->pid is not the blessed way to check whether a task is a group leader.

This is not about the code beautifulness only, but about pid namespaces fixes - both the tgid and the pid fields on the task_struct are (slowly :( ) becoming deprecated.

Besides, the thread_group_leader() macro makes only one dereference :)

---

index eb681a6..bc4cac6 100644
@@ -1065,7 +1065,7 @@ static void mem_cgroup_move_task(struct cgroup_subsys *ss,
 	 * Only thread group leaders are allowed to migrate, the mm_struct is
 	 * in effect owned by the leader
 	 */
-	if (p->tgid != p->pid)
+	if (!thread_group_leader(p))
 		goto out;
 
 	css_get(&mem->css);