src: add conntrack information to trace monitor mode

src: add conntrack information to trace monitor mode

Upcoming kernel change provides the packets conntrack state in the
trace message data.

This allows to see if packet is seen as original or reply, the conntrack
state (new, establieshed, related) and the status bits which show if e.g.
NAT was applied.  Alsoi include conntrack ID so users can use conntrack
tool to query the kernel for more information via ctnetlink.

This improves debugging when e.g. packets do not pick up the expected
NAT mapping, which could e.g. also happen because of expectations
following the NAT binding of the owning conntrack entry.

Example output ("conntrack: " lines are new):

trace id 32 t PRE_RAW packet: iif "enp0s3" ether saddr [..]
trace id 32 t PRE_RAW rule tcp flags syn meta nftrace set 1 (verdict continue)
trace id 32 t PRE_RAW policy accept
trace id 32 t PRE_MANGLE conntrack: ct direction original ct state new ct id 2641368242
trace id 32 t PRE_MANGLE packet: iif "enp0s3" ether saddr [..]
trace id 32 t ct_new_pre rule jump rpfilter (verdict jump rpfilter)
trace id 32 t PRE_MANGLE policy accept
trace id 32 t INPUT conntrack: ct direction original ct state new ct status dnat-done ct id 2641368242
trace id 32 t INPUT packet: iif "enp0s3" [..]
trace id 32 t public_in rule tcp dport 443 accept (verdict accept)

v3: remove clash bit again, kernel won't expose it anymore.
v2: add more status bits: helper, clash, offload, hw-offload.
    add flag explanation to documentation.

Signed-off-by: Florian Westphal <fw@strlen.de>
Reviewed-by: Pablo Neira Ayuso <pablo@netfilter.org>