cpudist: Protect against potentially negative time deltas

It seems from experimentation that the calculated timestamps between
on- and off-CPU switch events can produce incorrect results, with a
later event having a smaller timestamp. Discard events when the
resulting delta time would be negative.

tools/cpudist.py

@@ -82,6 +82,11 @@
  if (tsp == 0)
  return;
 
+ if (ts < *tsp) {
+ // Probably a clock issue where the recorded on-CPU event had a
+ // timestamp later than the recorded off-CPU event, or vice versa.
+ return;
+ }
  u64 delta = ts - *tsp;
  FACTOR
  STORE