bgpd: fix the IGP metric for best path selection on VPN import

bgpd: fix the IGP metric for best path selection on VPN import

Since the commit da0c0ef70c ("bgpd: VRF-Lite fix best path selection"),
the best path selection is made from the comparison of the attributes
of the original route i.e. the ultimate path.

The IGP metric is currently set on the child path instead of the
ultimate path (i.e. the parent path). On eBGP, the ultimate path is the
child path. However, for imported routes, the ultimate path is always
set to 0, which results in skipping the IGP metric comparison when
selecting the best path.

Set the IGP metric on the ultimate path when a BGP nexthop is added or
updated.

Fixes: da0c0ef70c ("bgpd: VRF-Lite fix best path selection")
Signed-off-by: Louis Scalbert <louis.scalbert@6wind.com>