zebra: across network namespace, vxlan remembers the link information

zebra: across network namespace, vxlan remembers the link information

the link information of vxlan interface is populated in layer 2
information, as well as in layer 2 vxlan information. This information
will be used later to collect vnis that are in other network namespaces,
but where bgp evpn is enabled on main network namespaces, and those vnis
have the link information in that namespace.

Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>