reparent_leader: check EXIT_DEAD instead of task_detached()

Change reparent_leader() to check ->exit_state instead of ->exit_signal,
this matches the similar EXIT_DEAD check in wait_consider_task() and
allows us to cleanup the do_notify_parent/task_detached logic.

task_detached() was really needed during reparenting before 9cd80bbb
"do_wait() optimization: do not place sub-threads on ->children list"
to filter out the sub-threads. After this change task_detached(p) can
only be true if p is the dead group_leader and its parent ignores
SIGCHLD, in this case the caller of do_notify_parent() is going to
reap this task and it should set EXIT_DEAD.

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