sizeof(struct ioapic_irqinfo) = 24
1 << IOAPIC_IRQI_SZSHIFT      = 16
INVARIANT BROKEN: ioapic_ipl.s (shll $16) and apic_vector.s (SIZE=24) index ioapic_irqs[] differently.
e.g. IRQ 100: ipl.s offset 1600 vs vector.s offset 2400 (array is under-walked)