netfilter: nf_conntrack: provide modparam to always register conntrack hooks

The connection tracking hooks can be optionally registered per netns
when conntrack is specifically invoked from the ruleset since
0c66dc1e ("netfilter: conntrack: register hooks in netns when needed
by ruleset"). Then, since 4d3a57f2 ("netfilter: conntrack: do not
enable connection tracking unless needed"), the default behaviour is
changed to always register them on demand.

This patch provides a toggle that allows users to always register them.
Without this toggle, in order to use conntrack for statistics
collection, you need a dummy rule that refers to conntrack, eg.

        iptables -I INPUT -m state --state NEW

This patch allows users to restore the original behaviour via modparam,
ie. always register connection tracking, eg.

        modprobe nf_conntrack enable_hooks=1

Hence, no dummy rule is required.

Reported-by: Laura Garcia <nevola@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>