sched_ext: Bypass BPF scheduler while PM events are in progress

PM operations freeze userspace. Some BPF schedulers have active userspace
component and may misbehave as expected across PM events. While the system
is frozen, nothing too interesting is happening in terms of scheduling and
we can get by just fine with the fallback FIFO behavior. Let's make things
easier by always bypassing the BPF scheduler while PM events are in
progress.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: David Vernet <dvernet@meta.com>