bgpd: prevent type-5 route creation if bgp_vrf->l3_vni is 0

bgpd: prevent type-5 route creation if bgp_vrf->l3_vni is 0

After a router reboot the L3 network via it converges before the L2
network. This is because MLAG intentionally holds down bridge-access
and vxlan-network ports for some time (MLAG init-delay) to prevent traffic
from switching to a router that is not fully ready. This also means that
routes (from vrf-peering sessions) that qualify for evpn type-5
advertisments are available long before the L3-VNI is available for that
tenant VRF. In these windows bgpd was adding these evpn-type-5 routes with
a L3-VNI of 0 (which was not fixed up after the L3-VNI became available) -

BGP routing table entry for 100.0.0.1:2:[5]:[0]:[0]:[32]:[200.1.1.1]
Paths: (1 available, best #1)
  Advertised to non peer-group peers:
  MSP1(uplink-1) MSP2(uplink-2)
  Route [5]:[0]:[0]:[32]:[200.1.1.1] VNI 0 >>>>>>>>
  65001 65535
    36.0.0.9 from 0.0.0.0 (27.0.0.9)
      Origin incomplete, metric 0, valid, sourced, local, bestpath-from-AS 65001, best
      Extended Community: ET:8 RT:5544:4001 Rmac:44:38:39:ff:ff:01
      AddPath ID: RX 0, TX 327
      Last update: Wed Feb 27 18:37:10 2019

Fix is to defer creating type-5 routes till the L3-VNI is available for
that tenant VRF (this was already being done for most cases; fixup takes
care of some that missed the check).

Ticket: CM-24022

Signed-off-by: Anuradha Karuppiah <anuradhak@cumulusnetworks.com>