zebra: single vxlan device vni handling

zebra: single vxlan device vni handling

This change brings in following functionality
- netlink_bridge_vxlan_vlan_vni_map_update for single vxlan devices
  This function is responsible for reading the vlan-vni map information
  received from netlink and populating a new hash_table with the vlan-vni
  data. Once all the vlan-vni data is collected, zebra_vxlan_if_vni_table_add_update
  is called to update vni_table in vxlan interface and process each of the
  vlan-vni data.
- refactoring changes for zevpn_build_hash_table
- existing zevpn_build_hash_table was walking over all the vxlan interfaces
  and then processing the vni for each of them. In case of single vxlan device,
  we will have more than one vni entries. This function is abstracted so that
  it iterates over all the vni entries for single vxlan device. For traditional
  vxlan device the zebra_vxlan_if_vni_iterate would only process single vni
  associated with that device.

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