bgpd: Check L3VNI status before announcing default

bgpd: Check L3VNI status before announcing default

Check that the L3VNI is "up" before taking action to announce or
withdraw the EVPN type-5 default based on configuration. Otherwise,
there can be timing conditions where a EVPN type-5 default route
gets announced without a VNI and with invalid route targets.

Signed-off-by: Vivek Venkatraman <vivek@nvidia.com>
Ticket: #2684144
Reviewed By: Chirag Shah
Testing Done:
1. Rerun failed test multiple times successfully
2. Some manual testing
3. precommit and partial evpn-smoke