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>
Signed-off-by: Dennis Cagle <d-cagle@codeaurora.org>
Git-repo: http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git
Git-commit: 6e94e0cfb0887e4013b3b930fa6ab1fe6bb6ba91
(cherry picked from commit 6e94e0cfb0887e4013b3b930fa6ab1fe6bb6ba91)
Change-Id: Idcfb2fffba72618a5fda1c6cc94394ed4f79be93