addrconf: refuse isatap eui64 for INADDR_ANY

A tunnel with no local ipv4 endpoint would otherwise use the
ISATAP linklocal address fe80::5efe:0:0, which is invalid. Rather not
add a linklocal address at all.

Signed-off-by: Sascha Hlusiak <contact@saschahlusiak.de>
Signed-off-by: David S. Miller <davem@davemloft.net>