sched_ext: RCU-protect the sub-sched tree's children/sibling lists

Future kfuncs need to walk descendants without scx_sched_lock. Make the
walker RCU-safe so that they can. A sub-sched's fields are initialized
before it is linked, so a walk that observes a linked node also observes its
setup. In-place changes after linking carry their own ordering.

Switch the children/sibling list ops to RCU and expand the descendant walker
to accept rcu_read_lock as a valid read-side context. Walkers that mutate
keep scx_sched_lock.

A sub-sched can be linked while an ancestor is bypassing, after the bypass
walk that propagates the depth has passed its parent. Bypass state is a
per-cpu flag plus a depth count and can't be established atomically at link
time, so refuse to link under a bypassing ancestor. Take scx_bypass_lock
across linking to check the parent's bypass state coherently.

v3: Reject linking under a bypassing ancestor instead of inheriting bypass_depth. (sashiko AI)
v2: Inherit bypass_depth before publishing @sch on the RCU sibling list.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Andrea Righi <arighi@nvidia.com>

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