*: EVPN symmetric routing for IPv6 tenant routes

*: EVPN symmetric routing for IPv6 tenant routes

Implement support for EVPN symmetric routing for IPv6 routes. The next hop
for EVPN routes is the IP address of the remote VTEP which is only an IPv4
address. This means that for IPv6 symmetric routing, there will be IPv6
destinations with IPv4 next hops. To make this work, the IPv4 next hops are
converted into IPv4-mapped IPv6 addresses.

As part of support, ensure that "L3" route-targets are not announced with
IPv6 link-local addresses so that they won't be installed in the routing
table.

Signed-off-by: Vivek Venkatraman vivek@cumulusnetworks.com
Reviewed-by: Mitesh Kanjariya mitesh@cumulusnetworks.com
Reviewed-by: Donald Sharp sharpd@cumulusnetworks.com