sched_ext: Factor out reenq_local() from scx_bpf_reenqueue_local()

sched_ext: Factor out reenq_local() from scx_bpf_reenqueue_local()

Factor out the core re-enqueue logic from scx_bpf_reenqueue_local() into a
new reenq_local() helper function. scx_bpf_reenqueue_local() now handles the
BPF kfunc checks and calls reenq_local() to perform the actual work.

This is a prep patch to allow reenq_local() to be called from other contexts.

Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>
Signed-off-by: Tejun Heo <tj@kernel.org>