zebra: Fix for mcast-group update and delete per vni for svd

zebra: Fix for mcast-group update and delete per vni for svd

Ticket: 2698649
Testing Done: precommit and evpn-min

Problem:
When the mcast-group is updated, the changes were being read from the netlink
and populated by zebra, but when kernel sends the delete of fdb delete for the
group, we are deleting the mcast-group that we newly updated. This is because,
currently we blindly reset the mcast-group during fdb delete without checking
for mcast-group associated to the vni.

Fix is to separate add/update and delete mcast-group functions and to check
for mcast-group before resetting during delete.

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