extensions: libip6t_frag: Add translation to nft

extensions: libip6t_frag: Add translation to nft

Add translation for frag to nftables. According to the --fraglen code:

	case O_FRAGLEN:
		/*
		 * As of Linux 3.0, the kernel does not check for
		 * fraglen at all.
		 */

In addition, the kernel code doesn't show any reference to the flag
IP6T_FRAG_LEN, so this option is deprecated and won't be translated to
nft.

Examples:

$ sudo iptables-translate -t filter -A INPUT -m frag --fragid 100:200 -j ACCEPT
nft add rule ip6 filter INPUT frag id 100-200 counter accept

$ sudo iptables-translate -t filter -A INPUT -m frag --fragid 100 --fragres --fragmore -j ACCEPT
nft add rule ip6 filter INPUT frag id 100 frag reserved 1 frag more-fragments 1 counter accept

$ sudo iptables-translate -t filter -A INPUT -m frag ! --fragid 100:200 -j ACCEPT
nft add rule ip6 filter INPUT frag id != 100-200 counter accept

$ sudo iptables-translate -t filter -A INPUT -m frag --fragid 100:200 --fraglast -j ACCEPT
nft add rule ip6 filter INPUT frag id 100-200 frag more-fragments 0 counter accept

$ sudo iptables-translate -t filter -A INPUT -m frag --fragid 100:200 --fragfirst -j ACCEPT
nft add rule ip6 filter INPUT frag id 100-200 frag frag-off 0 counter accept

$ sudo iptables-translate -t filter -A INPUT -m frag --fraglast -j ACCEPT
nft add rule ip6 filter INPUT frag more-fragments 0 counter accept

Signed-off-by: Laura Garcia Liebana <nevola@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>