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 inline bool unconditional(const struct ip6t_ip6 *ipv6)
{
	static const struct ip6t_ip6 uncond;

	return memcmp(ipv6, &uncond, sizeof(uncond)) == 0;
}

static inline bool unconditional(const struct ip6t_ip6 *ipv6)
{
	static const struct ip6t_ip6 uncond;

	return memcmp(ipv6, &uncond, sizeof(uncond)) == 0;
}
