zebra: remote RMAC for EVPN ipv6 hosts should be programmed against the ipv4 nexthop

zebra: remote RMAC for EVPN ipv6 hosts should be programmed against the ipv4 nexthop

For ipv6 host, the next hop is conevrted to ipv6 mapped address.
However, the remote rmac should still be programmed with the ipv4 address.
This is how the entries will look in the kernel for ipv6 hosts routing.

vrf routing table:
ipv6 -> ipv6_mapped remote vtep on l3vni SVI

neigh table:
ipv6_mapped remote vtep -> remote RMAC

bridge fdb:
remote rmac -> ipv4 vtep tunnel

Signed-off-by: Mitesh Kanjariya <mitesh@cumulusnetworks.com>