ipv4: Fix fib_trie build in some configurations.

If we end up including include/linux/node.h (either explicitly
or implicitly) that header has a definition of "structt node"
too.

So rename the one we use in fib_trie to "rt_trie_node" to avoid
the conflict.
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>