job control: Add @for_ptrace to do_notify_parent_cldstop()

Currently, do_notify_parent_cldstop() determines whether the
notification is for the real parent or ptracer.  Move the decision to
the caller by adding @for_ptrace parameter to
do_notify_parent_cldstop().  All the callers are updated to pass
task_ptrace(target_task), so this patch doesn't cause any behavior
difference.

While at it, add function comment to do_notify_parent_cldstop().

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Oleg Nesterov <oleg@redhat.com>