sched_ext: Drop TRACING access to select_cpu kfuncs

sched_ext: Drop TRACING access to select_cpu kfuncs

The select_cpu kfuncs - scx_bpf_select_cpu_dfl(), scx_bpf_select_cpu_and()
and __scx_bpf_select_cpu_and() - take task_rq_lock() internally. Exposing
them via scx_kfunc_set_idle to BPF_PROG_TYPE_TRACING is unsafe: arbitrary
tracing contexts (kprobes, tracepoints, fentry, LSM) may run with @p's
pi_lock state unknown.

Move them out of scx_kfunc_ids_idle into a new scx_kfunc_ids_select_cpu
set registered only for STRUCT_OPS and SYSCALL.

Extracted from a larger verifier-time kfunc context filter patch
originally written by Juntong Deng.

Original-patch-by: Juntong Deng <juntong.deng@outlook.com>
Cc: Cheng-Yang Chou <yphbchou0911@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Andrea Righi <arighi@nvidia.com>