zebra: vxlan interface handling changes

zebra: vxlan interface handling changes

This change modifies zebra_vxlan_if_up/down/add/update and del functionality
to be per vni based.

zebra_vxlan_if_add/update/del and zebra_vxlan_if_up/down now handles
the vni operations based on vxlan device type (single or traditional vxlan device).

zebra_vxlan_if_vni_table_add_update
- This function handles the vlan-vni map update received from the netlink
  interface to single vxlan device vni_table hash table.

zebra_vxlan_if_vni_mcast_group_update
- This function handles the new multicast group update received from
  the netlink interface to single vxlan device vni_table hash table.

For traditional vxlan interfaces, the vni and mcast group
handling follows the traditional approach.

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