netfilter: nfnetlink_cthelper: unbreak userspace helper support

Restore helper data size initialization and fix memcopy of the helper
data size.

Fixes: 157ffffeb5dc ("netfilter: nfnetlink_cthelper: reject too large userspace allocation requests")
Reviewed-by: Florian Westphal <[e-mail]>
Signed-off-by: Pablo Neira Ayuso <[e-mail]>

net/netfilter/nfnetlink_cthelper.c

···
 	if (help->helper->data_len == 0)
 		return -EINVAL;
 
-	nla_memcpy(help->data, nla_data(attr), sizeof(help->data));
+	nla_memcpy(help->data, attr, sizeof(help->data));
 	return 0;
 }
 
···
 		ret = -ENOMEM;
 		goto err2;
 	}
+	helper->data_len = size;
 
 	helper->flags |= NF_CT_HELPER_F_USERSPACE;
 	memcpy(&helper->tuple, tuple, sizeof(struct nf_conntrack_tuple));