[IPV4] fib_trie: fib_insert_node cleanup

The only error from fib_insert_node is if memory allocation fails, so
instead of passing by reference, just use the convention of returning
NULL.

Signed-off-by: Stephen Hemminger <stephen.hemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>