sched/cputime: Handle dyntick-idle steal time correctly

sched/cputime: Handle dyntick-idle steal time correctly

The dyntick-idle steal time is currently accounted when the tick restarts
but the stolen idle time is not subtracted from the idle time that was
already accounted. This is to avoid observing the idle time going backward
as the dyntick-idle cputime accessors can't reliably know in advance the
stolen idle time.

In order to maintain a forward progressing idle cputime while subtracting
idle steal time from it, keep track of the previously accounted idle stolen
time and substract it from _later_ idle cputime accounting.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Tested-by: Shrikanth Hegde <sshegde@linux.ibm.com>
Link: https://patch.msgid.link/20260508131647.43868-16-frederic@kernel.org