net: mpls: Delete route when all nexthops have been deleted

When all devices for all nexthops in a route have been deleted, the
route is effectively dead, so remove it.

Signed-off-by: David Ahern <dsa@cumulusnetworks.com>
Acked-by: Roopa Prabhu <roopa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>