zebra: update zl3vni when bridge link refreshed in other namespaces

zebra: update zl3vni when bridge link refreshed in other namespaces

When running bgp evpn rt5 setup with vrf namespace backend, once the
BGP configuration loaded, some refresh like the config change of a
vxlan interface is not taken into account. As consequence, the BGP
l2vpn evpn entries are empty. This can happen by recreating vxlan
interface like follows:

ip netns exec cust1 ip li del vxlan1000
ip link add vxlan1000 type vxlan id 1000 dev loopback0 local 10.209.36.1 learning
ip link set dev vxlan1000 mtu 9000
ip link set dev vxlan1000 netns cust1
ip netns exec cust1 bash
ip link set dev vxlan1000 up
ip link set dev vxlan1000 master br1000

Actually, changing learning attribute requires recreation, and this
change needs to manually reload the frr configuration.

The update mechanism in zebra about vxlan interface updates is
already put in place, but it does not work well with namespace
based vrf backend. The function zl3vni_from_svi() is then
modified to parse all the interfaces of each namespace.

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