netfilter: x_tables: make sure e->next_offset covers remaining blob size

Otherwise this function may read data beyond the ruleset blob.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
static int icmp_checkentry(const struct xt_mtchk_param *par)
{
	const struct ipt_icmp *icmpinfo = par->matchinfo;

	/* Must specify no unknown invflags */
	return (icmpinfo->invflags & ~IPT_ICMP_INV) ? -EINVAL : 0;
}

static int icmp_checkentry(const struct xt_mtchk_param *par)
{
	const struct ipt_icmp *icmpinfo = par->matchinfo;

	/* Must specify no unknown invflags */
	return (icmpinfo->invflags & ~IPT_ICMP_INV) ? -EINVAL : 0;
}
