xtables: Don't use native nftables comments

xtables: Don't use native nftables comments

The problem with converting libxt_comment into nftables comment is that
rules change when parsing from kernel due to comment match being moved
to the end of the match list. And since match ordering matters, the rule
may not be found anymore when checking or deleting. Apart from that,
iptables-nft didn't support multiple comments per rule anymore. This is
a compatibility issue without technical reason.

Leave conversion from nftables comment to libxt_comment in place so we
don't break running systems during an update.

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