The igb(4) logic for selecting CPUs doesn't start at cpuid 0 for each
NIC.  So igb0 gets cpu0-7, igb1 gets cpu8-15, etc.

Now, for RSS this won't (currently) work as there's no notion yet of
different RSS buckets -> CPU ID pools based on NIC.  So to line everything
up correctly, add in a hack to always start CPU assignment at CPU 0.
This means igb0 and igb1 both occupy CPU 0-7 in my test system, with
8-31 being free.  This, yes, is the RSS way.