act_ct: prepare for stolen verdict coming from conntrack and nat engine

At this time, conntrack either returns NF_ACCEPT or NF_DROP.
To improve debuging it would be nice to be able to replace NF_DROP verdict
with NF_DROP_REASON() helper,

This helper releases the skb instantly (so drop_monitor can pinpoint
exact location) and returns NF_STOLEN.

Prepare call sites to deal with this before introducing such changes
in conntrack and nat core.

Signed-off-by: Florian Westphal <fw@strlen.de>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>