cpudist: Use `finish_task_switch` kprobe instead of `sched_switch` tracepoint

cpudist: Use `finish_task_switch` kprobe instead of `sched_switch` tracepoint

The `sched_switch` tracepoint approach requires storing the previous
task's tgid in a map and fetching it from there, because it is not
available as a tracepoint argument. Instead, placing a kprobe on the
`finish_task_switch` function allows cleanly fetching the previous
task's pid and tgid from the task_struct.