net: sched: act_connmark: don't zap skb->nfct

This action is meant to be passive, i.e. we should not alter
skb->nfct: If nfct is present just leave it alone.

Compile tested only.

Cc: Jamal Hadi Salim <jhs@mojatatu.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Acked-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>