bgpd: make sure nh is valid for MPLS vpn routes

bgpd: make sure nh is valid for MPLS vpn routes

If we are using a nexthop for a MPLS VPN route make sure the
nexthop is over a labeled path. This new check mirrors the one
in validate_paths (where routes are enabled when a nexthop
becomes reachable). The check is introduced to the code path
where routes are added and the nexthop is looked up.

Signed-off-by: Pat Ruddy <pat@voltanet.io>