sched: KIST schedule() bad cast on monotime_diff_msec()

This code snippet creates a cast "overflow" from 64 bit to 32 bit:

  diff = (int32_t) monotime_diff_msec(&scheduler_last_run, &now);

Because at boot time, scheduler_last_run is set to 0 (static value), the diff that comes out is simply bigger than int32_t everytime leading to a diff value that is negative.

And then we would add the next run to libevent with a humongeous usec value (and remember that diff is big negative number):

next_run.tv_usec = (sched_run_interval - diff) * 1000;

Leading to libevent to fail to add the event and thus the scheduler never runs.

The fix here is to use int64_t and BUG() the event_add/active() so we can catch such issue in the future.