zebra: evpn update remote rmac and nexhop

zebra: evpn update remote rmac and nexhop

In symmetric routing case for evpn (type-2/type-5)
routes, nexthop and Router mac fields can change from
the originating VTEP.

At the receiving VTEP, bgp path info may points to different
nexthop IP (nh1->nh2) and Remote MAC remain the same.
When the bgp sync the route with nexthop and RMAC fields.
For the exisitng rmac entry update/replace with the new
nexthop (VTEP) IP in remote rmac db in Zebra.

Similarly, bgp path info may points different Router-mac
(RMAC1->RMAC2) and the nexthop value remains the same.
In this case, update to the new RMAC value for the
existing remote nexthop in the Zebra' nexthop cache db.

Ticket:CM-26917
Reviewed By:CCR-9435
Testing Done:

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