openvswitch: Fix comments for skb->_nfct

Fix comments referring to skb 'nfct' and 'nfctinfo' fields now that
they are combined into '_nfct'.

Signed-off-by: Jarno Rajahalme <jarno@ovn.org>
Acked-by: Pravin B Shelar <pshelar@ovn.org>
Acked-by: Joe Stringer <joe@ovn.org>
Signed-off-by: David S. Miller <davem@davemloft.net>