netfilter: nfnetlink_queue: fix timestamp attribute

Since 4.4 we erronously use timestamp of the netlink skb (which is zero).

Bugzilla: https://bugzilla.netfilter.org/show_bug.cgi?id=1066


Fixes: b28b1e82 ("netfilter: nfnetlink_queue: use y2038 safe timestamp")
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>