sched_ext: Prepare bypass mode for hierarchical operation

sched_ext: Prepare bypass mode for hierarchical operation

Bypass mode is used to simplify enable and disable paths and guarantee
forward progress when something goes wrong. When enabled, all tasks skip BPF
scheduling and fall back to simple in-kernel FIFO scheduling. While this
global behavior can be used as-is when dealing with sub-scheds, that would
allow any sub-sched instance to affect the whole system in a significantly
disruptive manner.

Make bypass state hierarchical by propagating it to descendants and updating
per-cpu flags accordingly. This allows an scx_sched to bypass if itself or
any of its ancestors are in bypass mode. However, this doesn't make the
actual bypass enqueue and dispatch paths hierarchical yet. That will be done
in later patches.

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