[NETLINK]: fib_lookup() via netlink

[NETLINK]: fib_lookup() via netlink

Below is a more generic patch to do fib_lookup via netlink. For others
we should say that we discussed this as a way to verify route selection.
It's also possible there are others uses for this.

In short the fist half of struct fib_result_nl is filled in by caller
and netlink call fills in the other half and returns it.

In case anyone is interested there is a corresponding user app to compare
the full routing table this was used to test implementation of the LC-trie.

Signed-off-by: David S. Miller <davem@davemloft.net>