Scheduler fixes:
- When stime is larger than rtime due to accounting imprecision, then
utime = rtime - stime becomes negative. As this is unsigned math, the
result becomes a huge positive number. Cure it by resetting stime to
rtime in that case, so utime becomes 0.
- Restore consistent state when sched_cpu_deactivate() fails.
When offlining a CPU fails in sched_cpu_deactivate() after the SMT
present counter has been decremented, then the function aborts but
fails to increment the SMT present counter and leaves it imbalanced.
Consecutive operations cause it to underflow. Add the missing fixup
for the error path.
As SMT accounting the runqueue needs to marked online again in the
error exit path to restore consistent state.