sched/core: Fix wakeup_preempt's next_class tracking

sched/core: Fix wakeup_preempt's next_class tracking

Kernel test robot reported that
tools/testing/selftests/kvm/hardware_disable_test was failing due to
commit 704069649b5b ("sched/core: Rework sched_class::wakeup_preempt()
and rq_modified_*()")

It turns out there were two related problems that could lead to a
missed preemption:

 - when hitting newidle balance from the idle thread, it would elevate
   rb->next_class from &idle_sched_class to &fair_sched_class, causing
   later wakeup_preempt() calls to not hit the sched_class_above()
   case, and not issue resched_curr().

   Notably, this modification pattern should only lower the
   next_class, and never raise it. Create two new helper functions to
   wrap this.

 - when doing schedule_idle(), it was possible to miss (re)setting
   rq->next_class to &idle_sched_class, leading to the very same
   problem.

Cc: Sean Christopherson <seanjc@google.com>
Fixes: 704069649b5b ("sched/core: Rework sched_class::wakeup_preempt() and rq_modified_*()")
Reported-by: kernel test robot <oliver.sang@intel.com>
Closes: https://lore.kernel.org/oe-lkp/202602122157.4e861298-lkp@intel.com
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20260218163329.GQ1395416@noisy.programming.kicks-ass.net