tracehook: fix SA_NOCLDWAIT

I outwitted myself again in commit 2b2a1ff64afbadac842bbc58c5166962cf4f7664,
and broke the SA_NOCLDWAIT behavior so it leaks zombies. This fixes it.

Reported-by: Andi Kleen <[e-mail]>
Signed-off-by: Roland McGrath <[e-mail]>

kernel/signal.c

···
 	struct siginfo info;
 	unsigned long flags;
 	struct sighand_struct *psig;
+	int ret = sig;
 
 	BUG_ON(sig == -1);
 
···
 		 * is implementation-defined: we do (if you don't want
 		 * it, just use SIG_IGN instead).
 		 */
-		tsk->exit_signal = -1;
+		ret = tsk->exit_signal = -1;
 		if (psig->action[SIGCHLD-1].sa.sa_handler == SIG_IGN)
 			sig = -1;
 	}
···
 	__wake_up_parent(tsk, tsk->parent);
 	spin_unlock_irqrestore(&psig->siglock, flags);
 
-	return sig;
+	return ret;
 }
 
 static void do_notify_parent_cldstop(struct task_struct *tsk, int why)