exit: reparent: fix the dead-parent PR_SET_CHILD_SUBREAPER reparenting

The ->has_child_subreaper code in find_new_reaper() finds alive "thread"
but returns another "reaper" thread which can be dead.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: Aaron Tomlin <atomlin@redhat.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Kay Sievers <kay@vrfy.org>
Cc: Lennart Poettering <lennart@poettering.net>
Cc: Sterling Alexander <stalexan@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/exit.c

@@ -512,7 +512,7 @@ static struct task_struct *find_new_reaper(struct task_struct *father)
 			thread = reaper;
 			do {
 				if (!(thread->flags & PF_EXITING))
-					return reaper;
+					return thread;
 			} while_each_thread(reaper, thread);
 		}
 	}