netfilter: nf_tables: allow NFPROTO_INET in nft_(match/target)_validate()

netfilter: nf_tables: allow NFPROTO_INET in nft_(match/target)_validate()

Commit d0009effa886 ("netfilter: nf_tables: validate NFPROTO_* family") added
some validation of NFPROTO_* families in the nft_compat module, but it broke
the ability to use legacy iptables modules in dual-stack nftables.

While with legacy iptables one had to independently manage IPv4 and IPv6
tables, with nftables it is possible to have dual-stack tables sharing the
rules. Moreover, it was possible to use rules based on legacy iptables
match/target modules in dual-stack nftables.

As an example, the program from [2] creates an INET dual-stack family table
using an xt_bpf based rule, which looks like the following (the actual output
was generated with a patched nft tool as the current nft tool does not parse
dual stack tables with legacy match rules, so consider it for illustrative
purposes only):

table inet testfw {
  chain input {
    type filter hook prerouting priority filter; policy accept;
    bytecode counter packets 0 bytes 0 accept
  }
}

After d0009effa886 ("netfilter: nf_tables: validate NFPROTO_* family") we get
EOPNOTSUPP for the above program.

Fix this by allowing NFPROTO_INET for nft_(match/target)_validate(), but also
restrict the functions to classic iptables hooks.

Changes in v3:
  * clarify that upstream nft will not display such configuration properly and
    that the output was generated with a patched nft tool
  * remove example program from commit description and link to it instead
  * no code changes otherwise

Changes in v2:
  * restrict nft_(match/target)_validate() to classic iptables hooks
  * rewrite example program to use unmodified libnftnl

Fixes: d0009effa886 ("netfilter: nf_tables: validate NFPROTO_* family")
Link: https://lore.kernel.org/all/Zc1PfoWN38UuFJRI@calendula/T/#mc947262582c90fec044c7a3398cc92fac7afea72
Link: https://lore.kernel.org/all/20240220145509.53357-1-ignat@cloudflare.com/
Reported-by: Jordan Griege <jgriege@cloudflare.com>
Signed-off-by: Ignat Korchagin <ignat@cloudflare.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>