use the internal index number as the unit number to compare with.
The first ethernet interface in Linux is called "eth0", whereas
our internal index starts wth 1 and is not unique to ethernet
interfaces (lo0 has index 1 for example). Instead, use a function-
local index number that starts with 0 and is incremented only
for ethernet interfaces. This way the unit number will match the
n-th ethernet interface in the system, which is exactly what it
means in Linux.
Tested by: Glenn Johnson <gjohnson@srrc.ars.usda.gov>
MFC after: 3 days
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