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.