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 04c19fa6f16047abff2288ddbc1f0798ede5a849
  ("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>