bgpd: check existing l3vni for any l2vni creation

bgpd: check existing l3vni for any l2vni creation

Scan all bgp vrf instances and respective L3VNI against the VNI which is being configured.

Ticket:CM-21859
Testing Done:
Configure l3vni,
try to configure same vni as l2vni under router bgp, address-family
l2vpn evpn.
The configuration is rejected.

show evpn vni
VNI        Type VxLAN IF              # MACs   # ARPs   # Remote VTEPs Tenant VRF
4001       L3   vx-4001               0        0        n/a vrf1

TOR(config)# router bgp 5546
TOR(config-router)# address-family l2vpn evpn
TOR(config-router-af)# vni 4001
% Failed to create VNI

Signed-off-by: Chirag Shah <chirag@cumulusnetworks.com>