zebra: avoid duplication node in l3vni l2vni-list

zebra: avoid duplication node in l3vni l2vni-list

With l2vni flap leading to duplicate entry creation
in l3vni's l2vni-list.
Use list sorted add with no duplicates.

root@TORC11:mgmt:~# show evpn vni 4001
VNI: 4001
  Type: L3
  Tenant VRF: vrf1
  State: Up
  ...
  L2 VNIs: 1000 1000 1000 0 0 1002
root@TORC11:mgmt:~# ip link set down vx-1002
root@TORC11:mgmt:~# ip link set up vx-1002
root@TORC11:mgmt:~# show evpn vni 4001
VNI: 4001
  Type: L3
  Tenant VRF: vrf1
  State: Up
  ...
  L2 VNIs: 1000 1000 1000 0 0 1002 1002

Ticket:CM-31545
Reviewed By:
Testing Done:

With Fix:
Multiple time flaps vni counts remained the same.

root@TORC11:mgmt:~# ip link set down vx-1002
root@TORC11:mgmt:~# ip link set up vx-1002
root@TORC11:mgmt:~# ip link set down vx-1002
root@TORC11:mgmt:~# ip link set up vx-1002
root@TORC11:mgmt:~# net show evpn vni 4001
VNI: 4001
  Type: L3
  Tenant VRF: vrf1
  State: Up
  ...
  L2 VNIs: 1000 1002

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