bgpd: fix label allocation per next-hop applied to unicast

bgpd: fix label allocation per next-hop applied to unicast

The label allocation per next-hop functionality is calling
the 'bgp_find_or_add_nexthop()' method using the SAFI_MPLS_VPN
safi parameter, whereas the call is supposed to apply to
unicast paths.

Fix this by using the SAFI_UNICAST safi parameter in the call.
Simplify the vpn_leak_from_vrf_get_per_nexthop_label() API by
removing the safi parameter from the function.

Fixes: 577be36a41be ("bgpd: add support for l3vpn per-nexthop label")
Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>