document de_thread() with exit_notify() connection

Add a couple of small comments, it is not easy to see what this code does.
Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Cc: Roland McGrath <roland@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/exec.c

@@ -798,7 +798,7 @@ static int de_thread(struct task_struct *tsk)
 	if (!thread_group_leader(tsk)) {
 		leader = tsk->group_leader;
 
-		sig->notify_count = -1;
+		sig->notify_count = -1;	/* for exit_notify() */
 		for (;;) {
 			write_lock_irq(&tasklist_lock);
 			if (likely(leader->exit_state))

kernel/exit.c

@@ -877,6 +877,7 @@ static void exit_notify(struct task_struct *tsk, int group_dead)
 		state = EXIT_DEAD;
 	tsk->exit_state = state;
 
+	/* mt-exec, de_thread() is waiting for us */
 	if (thread_group_leader(tsk) &&
 	    tsk->signal->notify_count < 0 &&
 	    tsk->signal->group_exit_task)