BGP: Fix nexthop setting for default originate

BGP: Fix nexthop setting for default originate

The nexthop setting for IPv6 default origination was incorrect, resulting in
the same (incorrect) IPv6 link-local nexthop being advertised to multiple
peers. The issue was also present for IPv4 default origination when IPv4
routes are advertised with IPv6 nextops (RFC 5549).

For default origination, we should just reset the nexthop when forming the
update for the update-group and let the outbound update/nexthop setting code
set it correctly.

Signed-off-by: Vivek Venkatraman <vivek@cumulusnetworks.com>
Reviewed-by:   Daniel Walton <dwalton@cumulusnetworks.com>
Reviewed-by:   Donald Sharp <sharpd@cumulusnetworks.com>
Reviewed-by:   Don Slice <dslice@cumulusnetworks.com>
Ticket: CM-10623
Reviewed By: CCR-4677
Testing Done: Manual, bgp-min