ipv4: fib_trie: Use tnode_get_child_rcu() and node_parent_rcu() in lookups

While looking for other fib_trie problems reported by Pawel Staszewski
I noticed there are a few uses of tnode_get_child() and node_parent()
in lookups instead of their rcu versions.

Signed-off-by: Jarek Poplawski <jarkao2@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>