selftests: netfilter: skip tests on early errors

br_netfilter: If we can't add the needed initial nftables ruleset skip the
test, kernel doesn't support a required feature.

rpath: run a subset of the tests if possible, but make sure we return
the skip return value so they are marked appropriately by the kselftest
framework.

nft_audit.sh: provide version information when skipping, this should
help catching kernel problem (feature not available in kernel) vs.
userspace issue (parser doesn't support keyword).

Signed-off-by: Florian Westphal <fw@strlen.de>
Link: https://lore.kernel.org/r/20240423130604.7013-7-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>