bgpd: correctly initialize the IP nexthop of redistributed routes

bgpd: correctly initialize the IP nexthop of redistributed routes

This is a preliminary work to export redistributed routes from
a given VRF in an VPN network. The exportation works well, when
the label allocation is based on an per-vrf mode, but not on
a per nexthop mode.

To associate a label with a connected nexthop, the nexthop
tracking contexts are used. Until today, there was no tracking
context for redistributed routes. But when using this vpn
allocation mode, one needs to know whether the route is directly
connected or not. When using the nexthop tracking context, the
nexthop attribute of the bgp update needs to have the nexthop
properly set. This was not the case for the mp_nexthop_global_in
attribute which was empty.

This commit is mandatory in order to later use nexthop tracking
context.

Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>