nft_set_pipapo: Generalise group size for buckets

Get rid of all hardcoded assumptions that buckets in lookup tables
correspond to four-bit groups, and replace them with appropriate
calculations based on a variable group size, now stored in struct
field.

The group size could now be in principle any divisor of eight. Note,
though, that lookup and get functions need an implementation
intimately depending on the group size, and the only supported size
there, currently, is four bits, which is also the initial and only
used size at the moment.

While at it, drop 'groups' from struct nft_pipapo: it was never used.
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>