netfilter: nfnetlink_queue: remove old clash resolution logic

For historical reasons there are two clash resolution spots in
netfilter, one in nfnetlink_queue and one in conntrack core.

nfnetlink_queue one was added first: If a colliding entry is found, NAT
NAT transformation is reversed by calling nat engine again with altered
tuple.

See commit 368982cd ("netfilter: nfnetlink_queue: resolve clash for
unconfirmed conntracks") for details.

One problem is that nf_reroute() won't take an action if the queueing
doesn't occur in the OUTPUT hook, i.e. when queueing in forward or
postrouting, packet will be sent via the wrong path.

Another problem is that the scenario addressed (2nd UDP packet sent with
identical addresses while first packet is still being processed) can also
occur without any nfqueue involvement due to threaded resolvers doing
A and AAAA requests back-to-back.

This lead us to add clash resolution logic to the conntrack core, see
commit 6a757c07 ("netfilter: conntrack: allow insertion of clashing
entries").  Instead of fixing the nfqueue based logic, lets remove it
and let conntrack core handle this instead.

Retain the ->update hook for sake of nfqueue based conntrack helpers.
We could axe this hook completely but we'd have to split confirm and
helper logic again, see commit ee04805f ("netfilter: conntrack: make
conntrack userspace helpers work again").

This SHOULD NOT be backported to kernels earlier than v5.6; they lack
adequate clash resolution handling.

Patch was originally written by Pablo Neira Ayuso.

Reported-by: Antonio Ojea <aojea@google.com>
Closes: https://bugzilla.netfilter.org/show_bug.cgi?id=1766


Signed-off-by: Florian Westphal <fw@strlen.de>
Tested-by: Antonio Ojea <aojea@google.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>