Currently, any address within the subnetwork will fallback on
loopback. It seems it has always been like that, but it seems wrong,
and I don't see a good reason to keep it this way. Fortunately, lack
of ARP reply made this unusable in practice, so we shouldn't break
much existing users.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>