x86/vector: Add a sanity check to prevent IRQ2 allocations

To prevent another incidental removal of the IRQ2 ignore logic in the
IO/APIC code going unnoticed add a sanity check. Add some commentry at the
other place which ignores IRQ2 while at it.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20210318192819.795280387@linutronix.de