bgpd: fix use nexthop tracking for exported vpn paths

bgpd: fix use nexthop tracking for exported vpn paths

When exporting redistributed prefixes from a given VRF
to an MPLS VPN network, the paths are always considered
as valid whereas it should not always be the case.

At exportation, a new MPLS VPN path is built in. Then
nexthop tracking is applied to the new path, and the
SAFI_MPLS_VPN parameter is used to tell the NHT code
to just check for the next-hop reachability. The previous
commit was wrongly considering that nexthop tracking was
never applied to mpls vpn networks. Ensure that nexthop
tracking for exported paths behaves as usual.

Fix this by not returning always 1 in the 'bgp_find_or_add_nexthop()'
function if the passed 'pi' parameter is a 'BGP_IMPORTED_ROUTE'
sub-type entry.

Fixes: 74be3f3ea9ec ("bgpd: track mpls vpn nexthops")
Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>