hrtimer: Rename hrtimer_cpu_base::in_hrtirq to deferred_rearm

hrtimer: Rename hrtimer_cpu_base::in_hrtirq to deferred_rearm

The upcoming deferred rearming scheme has the same effect as the deferred
rearming when the hrtimer interrupt is executing. So it can reuse the
in_hrtirq flag, but when it gets deferred beyond the hrtimer interrupt
path, then the name does not make sense anymore.

Rename it to deferred_rearm upfront to keep the actual functional change
separate from the mechanical rename churn.

Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20260224163430.935623347@kernel.org