zebra: svi down remove l2vni from l3vni list

zebra: svi down remove l2vni from l3vni list

Problem:
L2-VNI SVI down followed by L2-VNI's vxlan device
deletion leads to stale entry into L3VNI's
L2-VNI list.

Solution:
When L2-VNI associated SVI is down, default vrf
is the new tenant vrf.
Remove L2-VNI from L3VNI's l2vni list as
L3VNI/VRF is no longer valid in absence of associated
SVI.

When SVI is up re-add L2-VNI into associated VRF's
L3VNI.

The above remove/add from the L3VNI's L2VNI list is
already done when vxlan or L2-VNI is flaped, just need
to handle when SVI is flapped.

Ticket:#2817127
Reviewed By:
Testing Done:

After deleting SVI following by L2-VNI deletion,
L3VNI's L2-VNI list delets the L2-VNI. (no stale entry).

After adding back SVI/L2-VNI, L3VNI list adds back the
L2-VNI and it is associated right tenant VRF.

Signed-off-by: Chirag Shah <chirag@nvidia.com>