extensions: libxt_length: Add translation to nft

extensions: libxt_length: Add translation to nft

Add translation for module length to nftables.

Examples:

$ sudo iptables-translate -A INPUT -p icmp -m length --length 86:0xffff -j DROP
nft add rule ip filter INPUT ip protocol icmp meta length 86-65535 counter drop

$ sudo iptables-translate -A INPUT -p udp -m length --length :400
nft add rule ip filter INPUT ip protocol udp meta length 0-400 counter

$ sudo iptables-translate -A INPUT -p udp -m length --length 40
nft add rule ip filter INPUT ip protocol udp meta length 40 counter

$ sudo iptables-translate -A INPUT -p udp -m length ! --length 40
nft add rule ip filter INPUT ip protocol udp meta length != 40 counter

Signed-off-by: Shivani Bhardwaj <shivanib134@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>