ptrace: wait_task_zombie: s/->parent/->real_parent/

Change wait_task_zombie() to use ->real_parent instead of ->parent.  We
could even use current afaics, but ->real_parent is more clean.

We know that the child is not ptrace_reparented() and thus they are equal.
 But we should avoid using task_struct->parent, we are going to remove it.

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>