ofproto-dpif-trace: Improve NAT tracing.

ofproto-dpif-trace: Improve NAT tracing.

When ofproto/trace detects a recirc action it resumes execution at the
specified next table. However, if the ct action performs SNAT/DNAT,
e.g., ct(commit,nat(src=1.1.1.1:4000),table=42), the src/dst IPs and
ports in the oftrace_recirc_node->flow field are not updated. This leads
to misleading outputs from ofproto/trace as real packets would actually
first get NATed and might match different flows when recirculated.

Assume the first IP/port from the NAT src/dst action will be used by
conntrack for the translation and update the oftrace_recirc_node->flow
accordingly. This is not entirely correct as conntrack might choose a
different IP/port but the result is more realistic than before.

This fix covers new connections. However, for reply traffic that executes
actions of the form ct(nat, table=42) we still don't update the flow as
we don't have any information about conntrack state when tracing.

Also move the oftrace_recirc_node processing out of ofproto_trace()
and to its own function, ofproto_trace_recirc_node() for better
readability/

Signed-off-by: Dumitru Ceara <dceara@redhat.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>