extensions: libip6t_rt.c: Add translation to nft

extensions: libip6t_rt.c: Add translation to nft

Add translation for rt for options --rt-type, --rt-segsleft and --rt-len.

Examples:

$ sudo ip6tables-translate -A INPUT -m rt --rt-type 0 -j DROP
nft add rule ip6 filter INPUT rt type 0 counter drop

$ sudo ip6tables-translate -A INPUT -m rt ! --rt-len 22 -j DROP
nft add rule ip6 filter INPUT rt hdrlength != 22 counter drop

$ sudo ip6tables-translate -A INPUT -m rt --rt-segsleft 26 -j ACCEPT
nft add rule ip6 filter INPUT rt seg-left 26 counter accept

The xlate function returns 0 for other options.

Signed-off-by: Janani Ravichandran <janani.rvchndrn@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>