ptrace: mm_need_new_owner: use ->real_parent to search in the siblings

"Search in the siblings" should use ->real_parent, not ->parent.  If the
task is traced then ->parent == tracer, while the task's parent is always
->real_parent.

Signed-off-by: Oleg Nesterov <oleg@redhat.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

@@ -590,7 +590,7 @@ retry:
 	/*
 	 * Search in the siblings
 	 */
-	list_for_each_entry(c, &p->parent->children, sibling) {
+	list_for_each_entry(c, &p->real_parent->children, sibling) {
 		if (c->mm == mm)
 			goto assign_new_owner;
 	}