sched_ext: idle: Explicitly pass allowed cpumask to scx_select_cpu_dfl()

sched_ext: idle: Explicitly pass allowed cpumask to scx_select_cpu_dfl()

Modify scx_select_cpu_dfl() to take the allowed cpumask as an explicit
argument, instead of implicitly using @p->cpus_ptr.

This prepares for future changes where arbitrary cpumasks may be passed
to the built-in idle CPU selection policy.

This is a pure refactoring with no functional changes.

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