netfilter: nft_compat: do not dump private area

[ Upstream commit d701d8117200399d85e63a737d2e4e897932f3b6 ]

Zero pad private area, otherwise we expose private kernel pointer to
userspace. This patch also zeroes the tail area after the ->matchsize
and ->targetsize that results from XT_ALIGN().

Fixes: 0ca743a5 ("netfilter: nf_tables: add compatibility layer for x_tables")
Reported-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>