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

[ Upstream commit 6e94e0cfb0887e4013b3b930fa6ab1fe6bb6ba91 ]

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>
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>