netfilter: nf_ct_helper: disable automatic helper assignment

Four years ago we introduced a new sysctl knob to disable automatic
helper assignment in 72110dfaa907 ("netfilter: nf_ct_helper: disable
automatic helper assignment"). This knob kept this behaviour enabled by
default to remain conservative.

This measure was introduced to provide a secure way to configure
iptables and connection tracking helpers through explicit rules.

Give the time we have waited for this, let's turn off this by default
now, worse case users still have a chance to recover the former
behaviour by explicitly enabling this back through sysctl.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>