selftests: netfilter: conntrack_vrf.sh: prefer socat, not iperf3

Use socat, like most of the other scripts already do. This also makes
the script complete slightly faster (3s -> 1s).

iperf3 establishes two connections (1 control connection, and 1+x
depending on test), so adjust expected counter values as well.

Signed-off-by: Florian Westphal <fw@strlen.de>
Link: https://lore.kernel.org/r/20240423130604.7013-8-fw@strlen.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

selftests: netfilter: conntrack_vrf.sh: move to lib.sh infra

swap test for "ip" with "conntrack", former is already accounted for
via setup_ns helper. Also switch to bash.

Signed-off-by: Florian Westphal <fw@strlen.de>
Link: https://lore.kernel.org/r/20240411233624.8129-8-fw@strlen.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

selftests: netfilter: move to net subdir

.. so this can start re-using existing lib.sh infra in next patches.

Several of these scripts will not work, e.g. because they assume
rp_filter is disabled, or reliance on a particular version/flavor
of "netcat" tool.

Add config settings for them.

nft_trans_stress.sh script is removed, it also exists in the nftables
userspace selftests. I do not see a reason to keep two versions in
different repositories/projects.

The settings file is removed for now:

It was used to increase the timeout to avoid slow scripts from getting
zapped by the 45s timeout, but some of the slow scripts can be sped up.
Re-add it later for scripts that cannot be sped up easily.

Update MAINTAINERS to reflect that future updates to netfilter
scripts should go through netfilter-devel@.

Signed-off-by: Florian Westphal <fw@strlen.de>
Link: https://lore.kernel.org/r/20240411233624.8129-2-fw@strlen.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>