[NETFILTER]: ip6_tables: fixed conflicted optname for getsockopt

66 and 67 for getsockopt on IPv6 socket is doubly used for IPv6 Advanced
API and ip6tables. This moves numbers for ip6tables to 68 and 69.
This also kills XT_SO_* because {ip,ip6,arp}_tables doesn't have so much
common numbers now.

The old userland tools keep to behave as ever, because old kernel always
calls functions of IPv6 Advanced API for their numbers.

Signed-off-by: Yasuyuki Kozakai <yasuyuki.kozakai@toshiba.co.jp>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>