netlink: fix IPv6 prefix computation

netlink: fix IPv6 prefix computation

The prefix building algorithm in netlink phase was incorrect in
IPv6.

For example, when adding the following rule
 nft add rule ip6 nat postrouting ip6 saddr 2::/64 --debug=all

we had:

 ip6 nat postrouting 0 0
  [ payload load 16b @ network header + 8 => reg 1 ]
  [ bitwise reg 1 = (reg=1 & 0x00000000 0x99361540 0x00007f8d 0x2e33a1eb ) ^ 0x00000000 0x00000000 0x00000000 0x00000000 ]
  [ cmp eq reg 1 0x00000200 0x00000000 0x00000000 0x00000000 ]

With the patch the result is as expected:

 ip6 nat postrouting 0 0
  [ payload load 16b @ network header + 8 => reg 1 ]
  [ bitwise reg 1 = (reg=1 & 0xffffffff 0xffffffff 0x00000000 0x00000000 ) ^ 0x00000000 0x00000000 0x00000000 0x00000000 ]
  [ cmp eq reg 1 0x00000200 0x00000000 0x00000000 0x00000000 ]

Signed-off-by: Eric Leblond <eric@regit.org>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>