Make ng_netflow(9) use new routing KPI.

Make ng_netflow(9) use new routing KPI.

Netflow module is supposed to store (along with fields like
  gateway address and interface index) matched netmask for each record.
  This (currently) requires returning individual route entries, instead
  of optimized next-hop structure. Given that, use control-plane
  rib_lookup_info() function to avoid accessing rtentries directly.
While rib_lookup_info() might be slower, than fibX_lookup() flavours,
  it is more scalable than rtalloc1_fib(), because rtentry mutex is
  not acquired.