Add IP6T_SO_ORIGINAL_DST for Linux

[zonyitoo]

Target

• i686-unknown-linux-gnu
• x86_64-unknown-linux-gnu

References

// For IPv4, we can use SO_ORIGINAL_DST on SOL_IP level
ret = getsockopt(fd, SOL_IP, SO_ORIGINAL_DST, destaddr, &socklen);

// For IPv6, we should use IP6T_SO_ORIGINAL_DST on SOL_IPV6 level
ret = getsockopt(fd, SOL_IPV6, IP6T_SO_ORIGINAL_DST, destaddr, &socklen);

Defined in #include <linux/netfilter_ipv6/ip6_tables.h>

#define IP6T_SO_ORIGINAL_DST            80

[zonyitoo]

But actually, IP6T_SO_ORIGINAL_DST has exactly the same value as SO_ORIGINAL_DST.

https://patchwork.ozlabs.org/patch/195443/

We can use SO_ORIGINAL_DST for now, temporary.