netfilter: nfnetlink_cthelper: unbreak userspace helper support

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

Fixes: 157ffffe ("netfilter: nfnetlink_cthelper: reject too large userspace allocation requests")
Reviewed-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>