ipv4: fib_trie leaf free optimization

Avoid unneeded test in the case where object to be freed
has to be a leaf. Don't need to use the generic tnode_free()
function, instead just setup leaf to be freed.

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