sched_ext: idle: Allow scx_bpf_select_cpu_and() from unlocked context

Allow scx_bpf_select_cpu_and() to be used from an unlocked context, in
addition to ops.enqueue() or ops.select_cpu().

This enables schedulers, including user-space ones, to implement a
consistent idle CPU selection policy and helps reduce code duplication.

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