net: sched: fix error return code in tcf_del_walker()

When nla_put_u32() fails, 'ret' could be 0, it should
return error code in tcf_del_walker().

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>