sched_ext: idle: Accept an arbitrary cpumask in scx_select_cpu_dfl()

Many scx schedulers implement their own hard or soft-affinity rules
to support topology characteristics, such as heterogeneous architectures
(e.g., big.LITTLE, P-cores/E-cores), or to categorize tasks based on
specific properties (e.g., running certain tasks only in a subset of
CPUs).

Currently, there is no mechanism that allows to use the built-in idle
CPU selection policy to an arbitrary subset of CPUs. As a result,
schedulers often implement their own idle CPU selection policies, which
are typically similar to one another, leading to a lot of code
duplication.

To address this, modify scx_select_cpu_dfl() to accept an arbitrary
cpumask, that can be used by the BPF schedulers to apply the existent
built-in idle CPU selection policy to a subset of allowed CPUs.

With this concept the idle CPU selection policy becomes the following:
- always prioritize CPUs from fully idle SMT cores (if SMT is enabled),
- select the same CPU if it's idle and in the allowed CPUs,
- select an idle CPU within the same LLC, if the LLC cpumask is a
subset of the allowed CPUs,
- select an idle CPU within the same node, if the node cpumask is a
subset of the allowed CPUs,
- select an idle CPU within the allowed CPUs.

This functionality will be exposed through a dedicated kfunc in a
separate patch.

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