Hi,

We've come across some boards where the MADT will list IO APICs that 
don't really exist in hardware.

The current 2.6, the kernel will panic in io_apic_get_unique_id because 
it will fail to force set the APIC id.

This following patch adds a quick check when registering IO APICs to 
make sure they exist. We do this by actually checking to see if the data
register loads the version properly and by examining the index register
to see that it was actually modified from 0xff after the io_apic_read.

Signed-off-by: Mike Waychison <mikew-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>