ipv6: Allow the l3mdev to be a loopback

There is no way currently for an IPv6 client connect using a loopback
address in a VRF, whereas for IPv4 the loopback address can be added:

    $ sudo ip addr add dev vrfred 127.0.0.1/8
    $ sudo ip -6 addr add ::1/128 dev vrfred
    RTNETLINK answers: Cannot assign requested address

So allow ::1 to be configured on an L3 master device. In order for
this to be usable ip_route_output_flags needs to not consider ::1 to
be a link scope address (since oif == l3mdev and so it would be
dropped), and ipv6_rcv needs to consider the l3mdev to be a loopback
device so that it doesn't drop the packets.

Signed-off-by: Robert Shearman <rshearma@vyatta.att-mail.com>
Signed-off-by: Mike Manning <mmanning@vyatta.att-mail.com>
Reviewed-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/addrconf.c

@@ -997,6 +997,7 @@ ipv6_add_addr(struct inet6_dev *idev, struct ifa6_config *cfg,
 	if (addr_type == IPV6_ADDR_ANY ||
 	    addr_type & IPV6_ADDR_MULTICAST ||
 	    (!(idev->dev->flags & IFF_LOOPBACK) &&
+	     !netif_is_l3_master(idev->dev) &&
 	     addr_type & IPV6_ADDR_LOOPBACK))
 		return ERR_PTR(-EADDRNOTAVAIL);