[IPV4] fib_trie: cleanup

Try this out:
     * replace macro's with inlines
     * get rid of places doing multiple evaluations of NODE_PARENT

[akpm@linux-foundation.org: rcu_dereference wants an lval]

Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: David S. Miller <davem@davemloft.net>