sched_ext: switch class when preempted by higher priority scheduler

ops.cpu_release() function, if defined, must be invoked when preempted by
a higher priority scheduler class task. This scenario was skipped in
commit f422316d ("sched_ext: Remove switch_class_scx()"). Let's fix
it.

Fixes: f422316d ("sched_ext: Remove switch_class_scx()")
Signed-off-by: Honglei Wang <jameshongleiwang@126.com>
Acked-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>