netfilter: ipset: deduplicate prefixlen maps

The prefixlen maps used here are identical, and have been since
introduction.  It seems to make sense to use a single large map,
that the preprocessor will fill appropriately.
Signed-off-by: Aaron Conole <aconole@bytheb.org>
Signed-off-by: Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>