cpusets: fix obsolete comment

mm migration is no longer done in cpuset_update_task_memory_state() so it
can no longer take current->mm->mmap_sem, so fix the obsolete comment.

[ This changed in commit 04c19fa
  ("cpuset: migrate all tasks in cpuset at once") when the mm migration
  was moved from cpuset_update_task_memory_state() to update_nodemask() ]

Signed-off-by: David Rientjes <rientjes@google.com>
Cc: Paul Jackson <pj@sgi.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/cpuset.c

@@ -322,8 +322,8 @@ static void guarantee_online_mems(const struct cpuset *cs, nodemask_t *pmask)
  * Call without callback_mutex or task_lock() held.  May be
  * called with or without cgroup_mutex held.  Thanks in part to
  * 'the_top_cpuset_hack', the task's cpuset pointer will never
- * be NULL.  This routine also might acquire callback_mutex and
- * current->mm->mmap_sem during call.
+ * be NULL.  This routine also might acquire callback_mutex during
+ * call.
  *
  * Reading current->cpuset->mems_generation doesn't need task_lock
  * to guard the current->cpuset derefence, because it is guarded