[NETFILTER]: nf_conntrack: use jhash2 in __hash_conntrack

Now it uses jhash, but using jhash2 would be around 3-4 times faster
(on P4).

Signed-off-by: Sami Farin <safari-netfilter@safari.iki.fi>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>