applied to tip/irq/sparseirq, thanks.
Maybe some design documentation would be nice too, to point out that
most of the pain we do here is because we dont have bootmem available as
early as we need an already instantiated dyn-array based IRQ array.
So we try this mix of an abstraction of preallocated and post-allocated
array, with pre-bootmem and post-bootmem behavior.