Routing code relies on stable ifa's during route creation.
This is needed due to the fact that nhop objects stores (referenced) ifa object for the whole nhop object lifetime.
To avoid the situation when ifa gets scheduled for the deleted during the route addition process, rt_getifa_fib() references found ifa as soon as possible.
Plug refcount leak by freeing ifa reference immediately after creating nexthop, regardless of the result.