netfilter: ctnetlink: fix timeout calculation

The sanity check (timeout < 0) never works; the dividend is unsigned
and so is the division, which should have been a signed division.

	long timeout = (ct->timeout.expires - jiffies) / HZ;
	if (timeout < 0)
		timeout = 0;

This patch converts the time values to signed for the division.

Signed-off-by: Xi Wang <xi.wang@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>