netfilter: nf_tables: avoid skb access on nf_stolen

In the Linux kernel, the following vulnerability has been resolved:

netfilter: nf_tables: avoid skb access on nf_stolen

When verdict is NF_STOLEN, the skb might have been freed.

When tracing is enabled, this can result in a use-after-free:

1. access to skb->nf_trace

2. access to skb->mark

3. computation of trace id

4. dump of packet payload

To avoid 1, keep a cached copy of skb->nf_trace in the

trace state struct.

Refresh this copy whenever verdict is != STOLEN.

Avoid 2 by skipping skb->mark access if verdict is STOLEN.

3 is avoided by precomputing the trace id.

Only dump the packet when verdict is not "STOLEN".