zebra: del auto mac when vni is down

zebra: del auto mac when vni is down

Delete an auto MAC with no neighbor associated,
when its VNI is down.

In Following sequence stale MAC entry retained in
FRR (zebra).
- Local MAC-IP pair
- MAC is deleted in bridge fdb table
- VNI is down, triggers IP (neigh) entries removed
from FRR DB.
- MAC retained as AUTO MAC with neigh list count 0.
- When VNI is UP again, stale MAC entry retained in FRR
DB.
When the MAC-IP pair moves behind remote VTEP, local VTEP
fails to add remote entry as its MAC is in auto state.

Ticket:CM-25504
Reviewed By:
Testing Done:

Validated the sequence with fix and auto MAC is deleted
when VNI is down.
When VNI comes up, the remote MAC-IP is added to FRR (Zebra)
and kernel.

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