io_apic: Don't route to APIC ID > 255

[cperciva]

Summary

I/O APIC Redirection Table Entries use 8 bits encode the Destination ID.
Attempting to route an IRQ to a higher APIC ID would result in it being
silently routed to the value reduced modulo 256, causing a panic if the
IRQ fired since the receiving CPU would not expect that IRQ.

Instead, print an error message and mask the interrupt. Note that we
also check io_cpu in ioapic_enable_source and force the mask bit back
on despite the request to remove it.

Tested on: EC2 r8i.96xlarge
MFC after: 1 month
Sponsored by: Amazon

[cperciva]

Add comment about keeping interrupt masked even when "enabling" it.

[kib]

Did you tried enabling DMAR/AMD IOMMU and interrupt remapping on this machine?

[cperciva]

In D55857#1278564, @kib wrote:

Did you tried enabling DMAR/AMD IOMMU and interrupt remapping on this machine?

This (virtual) machine doesn't have a proper IOMMU. See http://david.woodhou.se/15-bit-msi.pdf for context.