sched_ext: Pin parent scx_sched across a child sub-scheduler's lifetime

sched_ext: Pin parent scx_sched across a child sub-scheduler's lifetime

A child sub-scheduler dereferences its parent scx_sched throughout its life,
e.g., in scx_sub_disable() which reparents the child's tasks and calls
parent->ops.sub_detach() after unlinking from the parent. However, the
parent is pinned only through parent->sub_kset, which is dropped during
disable. The parent scx_sched can be RCU-freed while a child is still
disabling.

Take a direct reference on the parent in scx_alloc_and_add_sched(), dropped
in scx_sched_free_rcu_work(), so a parent always outlives its descendants.

Signed-off-by: Tejun Heo <tj@kernel.org>