netfilter: conntrack: dccp: try not to drop skb in conntrack

It would be better not to drop skb in conntrack unless we have good
alternatives. So we can treat the result of testing skb's header
pointer as nf_conntrack_tcp_packet() does.

Signed-off-by: Jason Xing <kernelxing@tencent.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>