netfilter: ebtables: CONFIG_COMPAT: don't trust userland offsets

We need to make sure the offsets are not out of range of the
total size.
Also check that they are in ascending order.

The WARN_ON triggered by syzkaller (it sets panic_on_warn) is
changed to also bail out, no point in continuing parsing.

Briefly tested with simple ruleset of
-A INPUT --limit 1/s' --log
plus jump to custom chains using 32bit ebtables binary.

Reported-by: <syzbot+845a53d13171abf8bf29@syzkaller.appspotmail.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
void ebt_unregister_table(struct net *net, struct ebt_table *table,
			  const struct nf_hook_ops *ops)
{
	if (ops)
		nf_unregister_net_hooks(net, ops, hweight32(table->valid_hooks));
	__ebt_unregister_table(net, table);
}

void ebt_unregister_table(struct net *net, struct ebt_table *table,
			  const struct nf_hook_ops *ops)
{
	if (ops)
		nf_unregister_net_hooks(net, ops, hweight32(table->valid_hooks));
	__ebt_unregister_table(net, table);
}
