netfilter: ip6t_NPT: Fix translation for non-multiple of 32 prefix lengths

The bitmask used for the prefix mangling was being calculated
incorrectly, leading to the wrong part of the address being replaced
when the prefix length wasn't a multiple of 32.

Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>