ethdev: introduce if_index in device info

ethdev: introduce if_index in device info

This field is intended for pcap to describe the name of the interface
as known to Linux. It is an interface index, but can be translated into
an interface name using if_indextoname() function.

When using pcap, interrupt affinity becomes important, and this field
gives the application a chance to ensure that interrupt affinity is set
to the lcore handling the device.

Signed-off-by: Mats Liljegren <mats.liljegren@enea.com>
Reviewed-by: Stephen Hemminger <stephen@networkplumber.org>
Acked-by: Thomas Monjalon <thomas.monjalon@6wind.com>