sched: Optimize hrtimer handling

schedule() provides several mechanisms to update the hrtick timer:

  1) When the next task is picked

  2) When the balance callbacks are invoked before rq::lock is released

Each of them can result in a first expiring timer and cause a reprogram of
the clock event device.

Solve this by deferring the rearm to the end of schedule() right before
releasing rq::lock by setting a flag on entry which tells hrtick_start() to
cache the runtime constraint in rq::hrtick_delay without touching the timer
itself.

Right before releasing rq::lock evaluate the flags and either rearm or
cancel the hrtick timer.

Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20260224163429.273068659@kernel.org

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