netfilter: nft_set_pipapo: .walk does not deal with generations

The .walk callback iterates over the current active set, but it might be
useful to iterate over the next generation set. Use the generation mask
to determine what set view (either current or next generation) is use
for the walk iteration.

Fixes: 3c4287f6

 ("nf_tables: Add set type for arbitrary concatenation of ranges")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>