If we get a stray interrupt, return after logging it. In the extremely
rare case of a stray interrupt to an unregistered source (such as a stray
interrupt from the 8259As when using APIC), this could result in a page
fault when it tried to walk the list of interrupt handlers to execute
INTR_FAST handlers. This bug was introduced with the intr_event changes,
so it's not present in 5.x or 6.x.

Submitted by: Mark Tinguely tinguely at casselton dot net