sched/cputime: Handle idle irqtime gracefully

The dyntick-idle cputime accounting always assumes that interrupt time
accounting is enabled and consequently stops elapsing the idle time during
dyntick-idle interrupts.

This doesn't mix up well with disabled interrupt time accounting because
then idle interrupts become a cputime blind-spot. Also this feature is
disabled on most configurations and the overhead of pausing dyntick-idle
accounting while in idle interrupts could then be avoided.

Fix the situation with conditionally pausing dyntick-idle accounting during
idle interrupts only iff either native vtime (which does interrupt time
accounting) or generic interrupt time accounting are enabled.

Also make sure that the accumulated interrupt time is not accidentally
substracted from later 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/20260508131647.43868-15-frederic@kernel.org

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