sched_ext: Validate prev_cpu in scx_bpf_select_cpu_dfl()

In the Linux kernel, the following vulnerability has been resolved:

sched_ext: Validate prev_cpu in scx_bpf_select_cpu_dfl()

If a BPF scheduler provides an invalid CPU (outside the nr_cpu_ids

range) as prev_cpu to scx_bpf_select_cpu_dfl() it can cause a kernel

crash.

To prevent this, validate prev_cpu in scx_bpf_select_cpu_dfl() and

trigger an scx error if an invalid CPU is specified.