do not change dead_task->exit_signal

__ptrace_detach() and do_notify_parent() set task->exit_signal = -1
to mark the task dead. This is no longer needed, nobody checks
exit_signal to detect the EXIT_DEAD task.

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

kernel/ptrace.c

@@ -384,7 +384,6 @@ static bool __ptrace_detach(struct task_struct *tracer, struct task_struct *p)
 			dead = do_notify_parent(p, p->exit_signal);
 		else if (ignoring_children(tracer->sighand)) {
 			__wake_up_parent(p, tracer);
-			p->exit_signal = -1;
 			dead = true;
 		}
 	}