cgroup: remove redundant get/put of task struct

threadgroup_lock() guarantees that the target threadgroup will
remain stable - no new task will be added, no new PF_EXITING
will be set and exec won't happen.

Changes in V2:
* https://lkml.org/lkml/2011/12/20/369 (Tejun Heo)
  * Undo incorrect removal of get/put from attach_task_by_pid()
* Author
  * Remove a comment which is made stale by this change

Signed-off-by: Mandeep Singh Baines <msb@chromium.org>
Acked-by: Li Zefan <lizf@cn.fujitsu.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: containers@lists.linux-foundation.org
Cc: cgroups@vger.kernel.org
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Paul Menage <paul@paulmenage.org>