[IPV4] nl_fib_lookup: Initialise res.r before fib_res_put(&res)

When CONFIG_IP_MULTIPLE_TABLES is enabled, the code in nl_fib_lookup()
needs to initialize the res.r field before fib_res_put(&res) - unlike
fib_lookup(), a direct call to ->tb_lookup does not set this field.

Signed-off-by: Sergey Vlasov <vsu@altlinux.ru>
Signed-off-by: David S. Miller <davem@davemloft.net>