zebra: send a local-mac del to bgpd on mac mod to remote

zebra: send a local-mac del to bgpd on mac mod to remote

When events cross paths between bgp and zebra bgpd could end up with a
dangling local MAC entry. Consider the following sequence of events on
rack-1 -
1. MAC1 has MM sequence number 1 and points to rack-3
2. Now a packet is rxed locally on rack-1 and rack-2 (simultaneously) with
source-mac=MAC1.
3. This would cause rack-1 and rack-2 to set the MM seq to 2 and
simultaneously report the MAC as local.
4. Now let's say on rack-1 zebra's MACIP_ADD is in bgpd's queue. bgpd
accepts rack-3's update and sends a remote MACIP add to zebra with MM=2.
5. zebra updates the MAC entry from local=>remote.
6. bgpd now processes zebra's "stale local" making it the best path.
However zebra no longer has a local MAC entry.

At this point bgpd and zebra are effectively out of sync i.e. bgpd has a
local-MAC which is not present in the kernel or in zebra.

To handle this window zebra should send a local MAC delete to bgpd on
modifying its cache to remote.

Ticket: CM-22687
Reviewed By: CCR-7935

Signed-off-by: Anuradha Karuppiah <anuradhak@cumulusnetworks.com>