netfilter: ip6t_NPT: Ensure to check lower part of prefixes are zero

RFC 6296 points that address bits that are not part of the prefix
has to be zeroed.

Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>