netfilter: ipv6: add getsockopt to retrieve origdst

userspace can query the original ipv4 destination address of a REDIRECTed
connection via
getsockopt(m_sock, SOL_IP, SO_ORIGINAL_DST, &m_server_addr, &addrsize)

but for ipv6 no such option existed.

This adds getsockopt(..., IPPROTO_IPV6, IP6T_SO_ORIGINAL_DST, ...).

Without this, userspace needs to parse /proc or use ctnetlink, which
appears to be overkill.

This uses option number 80 for IP6T_SO_ORIGINAL_DST, which is spare,
to use the same number we use in the IPv4 socket option SO_ORIGINAL_DST.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

include/uapi/linux/in6.h

@@ -240,6 +240,7 @@ struct in6_flowlabel_req {
  *
  * IP6T_SO_GET_REVISION_MATCH	68
  * IP6T_SO_GET_REVISION_TARGET	69
+ * IP6T_SO_ORIGINAL_DST		80
  */
 
 /* RFC5014: Source address selection */