zebra: Handle vni determination for non-vlan-aware bridges

zebra: Handle vni determination for non-vlan-aware bridges

This patch addresses following

- Remove unused VLAN Id parameter when trying to determine the VNI associated
  with a non-VLAN aware bridge. Also, add a check to ensure that in this case,
  we have a per-VNI VXLAN interface. Due to sequence of events, it is possible
  that we may have VLAN-VNI mappings, in which case the code should return
  gracefully.

- With support for a container VXLAN interface that has VLAN-VNI mappings,
  the VXLAN interface itself may be up but a particular VNI might have
  been removed. Ensure that VNI mapping exists before proceeding with
  further processing.

Signed-off-by: Sharath Ramamurthy <sramamurthy@nvidia.com>