Some IVAR indices are special in that they have global meaning across
multiple buses where as other IVARs are always private to the local bus.
Try to document this a bit and add constants for the various ranges to
avoid future conflicts.
This is a no-op, but IVAR indices are now generally defined as enums
as that makes it easier to define them in terms of ranges.
Reviewed by: imp, royger, andrew
Differential Revision: https://reviews.freebsd.org/D54159
John Baldwin
6cf4e30252