When an interface doesn't have a layer 2 address in6_get_ifid()

When an interface doesn't have a layer 2 address in6_get_ifid()
tries to "borrow" one from another interface.
But then it checks if the U bit is set int the generated EUI64
address and rejects it.
On the other hand for interfaces that do have a layer 2 address this
check is skipped, so relax it for the "borrowing" case, too.

With this one gets stable link local addresses on e.g. gre(4)
interfaces on certain virtualisation environments depending which mac
addresses get picked for the vio(4) interfaces while previously we
would end up with a random IP on every reboot.

Reported by Aaron A. Glenn via phessler.

OK phessler, benno