exit: Guarantee make_task_dead leaks the tsk when calling do_task_exit

Change the task state to EXIT_DEAD and take an extra rcu_refernce
to guarantee the task will not be reaped and that it will not be
freed.

Link: https://lkml.kernel.org/r/YdUzjrLAlRiNLQp2@zeniv-ca.linux.org.uk


Pointed-out-by: Al Viro <viro@zeniv.linux.org.uk>
Fixes: 7f80a2fd ("exit: Stop poorly open coding do_task_dead in make_task_dead")
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>