netfilter: ebtables: do not hook tables by default

If any of these modules is loaded, hooks get registered in all netns:

Before: 'unshare -n nft list hooks' shows:
family bridge hook prerouting {
	-2147483648 ebt_broute
	-0000000300 ebt_nat_hook
}
family bridge hook input {
	-0000000200 ebt_filter_hook
}
family bridge hook forward {
	-0000000200 ebt_filter_hook
}
family bridge hook output {
	+0000000100 ebt_nat_hook
	+0000000200 ebt_filter_hook
}
family bridge hook postrouting {
	+0000000300 ebt_nat_hook
}

This adds 'template 'tables' for ebtables.

Each ebtable_foo registers the table as a template, with an init function
that gets called once the first get/setsockopt call is made.

ebtables core then searches the (per netns) list of tables.
If no table is found, it searches the list of templates instead.
If a template entry exists, the init function is called which will
enable the table and register the hooks (so packets are diverted
to the table).

If no entry is found in the template list, request_module is called.

After this, hook registration is delayed until the 'ebtables'
(set/getsockopt) request is made for a given table and will only
happen in the specific namespace.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>