xtables-translate: Avoid querying the kernel

xtables-translate: Avoid querying the kernel

This originally came up when accidentally calling iptables-translate as
unprivileged user - nft_compatible_revision() then fails every time,
making the translator fall back to using revision 0 only which often
leads to failed translations (due to missing xlate callback).

The bottom line is there is no need to check what revision of a given
iptables match the kernel supports when it is only to be translated into
an nftables equivalent. So just assign a dummy callback returning good
for any revision being asked for.

Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>