exit: add lock context annotation on find_new_reaper()

find_new_reaper() releases and regrabs tasklist_lock but was missing
proper annotations.  Add it.  This remove following sparse warning:

 warning: context imbalance in 'find_new_reaper' - unexpected unlock
Signed-off-by: Namhyung Kim <namhyung@gmail.com>
Acked-by: Roland McGrath <roland@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/exit.c

@@ -703,6 +703,8 @@ static void exit_mm(struct task_struct * tsk)
  * space.
  */
 static struct task_struct *find_new_reaper(struct task_struct *father)
+	__releases(&tasklist_lock)
+	__acquires(&tasklist_lock)
 {
 	struct pid_namespace *pid_ns = task_active_pid_ns(father);
 	struct task_struct *thread;