gre: add link local route when local addr is any

This bug was reported by Steinar H. Gunderson and was introduced by commit
f7cb8886 ("sit/gre6: don't try to add the same route two times").

root@morgental:~# ip tunnel add foo mode gre remote 1.2.3.4 ttl 64
root@morgental:~# ip link set foo up mtu 1468
root@morgental:~# ip -6 route show dev foo
fe80::/64  proto kernel  metric 256

but after the above commit, no such route shows up.

There is no link local route because dev->dev_addr is 0 (because local ipv4
address is 0), hence no link local address is configured.

In this scenario, the link local address is added manually: 'ip -6 addr add
fe80::1 dev foo' and because prefix is /128, no link local route is added by the
kernel.

Even if the right things to do is to add the link local address with a /64
prefix, we need to restore the previous behavior to avoid breaking userpace.

Reported-by: Steinar H. Gunderson <sesse@samfundet.no>
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>