bgpd: use nexthop interface when adding LSP in BGP MPLSVPN

bgpd: use nexthop interface when adding LSP in BGP MPLSVPN

BGP MPLSVPN next hop label allocation was using only the next-hop
IP address. As MPLSVPN contexts rely on bnc contexts, the real
nexthop interface is known, and the LSP entry to enter can apply
to the specific interface. To illustrate, the BGP service is able
to handle the following two iproute2 commands:

 > ip -f mpls route add 105 via inet 192.0.2.45 dev r1-eth1
 > ip -f mpls route add 105 via inet 192.0.2.46 dev r1-eth2

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