exit_notify: fix kill_orphaned_pgrp() usage with mt exit

1. exit_notify() always calls kill_orphaned_pgrp(). This is wrong, we
   should do this only when the whole process exits.

2. exit_notify() uses "current" as "ignored_task", obviously wrong.
   Use ->group_leader instead.

Test case:

	void hup(int sig)
	{
		printf("HUP received\n");
	}

	void *tfunc(void *arg)
	{
		sleep(2);
		printf("sub-thread exited\n");
		return NULL;
	}

	int main(int argc, char *argv[])
	{
		if (!fork()) {
			signal(SIGHUP, hup);
			kill(getpid(), SIGSTOP);
			exit(0);
		}

		pthread_t thr;
		pthread_create(&thr, NULL, tfunc, NULL);

		sleep(1);
		printf("main thread exited\n");
		syscall(__NR_exit, 0);

		return 0;
	}

output:

	main thread exited
	HUP received
	Hangup

With this patch the output is:

	main thread exited
	sub-thread exited
	HUP received
Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/exit.c

@@ -750,7 +750,7 @@ static void forget_original_parent(struct task_struct *father)
  * Send signals to all our closest relatives so that they know
  * to properly mourn us..
  */
-static void exit_notify(struct task_struct *tsk)
+static void exit_notify(struct task_struct *tsk, int group_dead)
 {
 	int state;
 
@@ -766,7 +766,8 @@ static void exit_notify(struct task_struct *tsk)
 	exit_task_namespaces(tsk);
 
 	write_lock_irq(&tasklist_lock);
-	kill_orphaned_pgrp(tsk, NULL);
+	if (group_dead)
+		kill_orphaned_pgrp(tsk->group_leader, NULL);
 
 	/* Let father know we died
 	 *
@@ -981,7 +982,7 @@ NORET_TYPE void do_exit(long code)
 		module_put(tsk->binfmt->module);
 
 	proc_exit_connector(tsk);
-	exit_notify(tsk);
+	exit_notify(tsk, group_dead);
 #ifdef CONFIG_NUMA
 	mpol_free(tsk->mempolicy);
 	tsk->mempolicy = NULL;