netlink: handle veth loops correctly

netlink: handle veth loops correctly

Since Linux 4.1, a pair of veth are referenced as IFLINK for each
other. We previously detected this loop easily and did break the loop by
removing this bogus information. However, when one of the interface is
updated, only the modified interface has IFLINK pointing to the other
interface. Therefore, no more loop but the information is incorrect. To
avoid that, don't reset indexes, just pointers. We only use indexes to
build correctly those pointers.