zebra: Bug fixes in fdb read for flooded traffic and remote fdb cleanup upon vni...

zebra: Bug fixes in fdb read for flooded traffic and remote fdb cleanup upon vni removal

This patch addresses following issues,
- When the VLAN-VNI mapping is configured via a map and not using
  individual VXLAN interfaces, upon removal of a VNI ensure that the
  remote FDB entries are uninstalled correctly.

- When VNI configuration is performed using VLAN-VNI mapping (i.e., without
  individual VXLAN interfaces) and flooded traffic is handled via multicast,
  the multicast group corresponding to the VNI needs to be explicitly read
  from the bridge FDB. This is relevant in the case of netlink interface to
  the kernel and for the scenario where a new VNI is provisioned or comes up.

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