Only use the MULTIPATH attribute when adding routes with more than one
next hop.
This solves issues with two scenarios:
1. Adding an IPv4 route to a kernel configured without
CONFIG_IP_ROUTE_MULTIPATH=y.
2. Adding an IPv6 route in general, since the MULTIPATH attribute is not
supported there.
Signed-off-by: Michael Altizer <xiche@verizon.net>