netfilter: nft_set_pipapo: allocate pcpu scratch maps on clone

This is needed in case a new transaction is made that doesn't insert any
new elements into an already existing set.

Else, after second 'nft -f ruleset.txt', lookups in such a set will fail
because ->lookup() encounters raw_cpu_ptr(m->scratch) == NULL.

For the initial rule load, insertion of elements takes care of the
allocation, but for rule reloads this isn't guaranteed: we might not
have additions to the set.

Fixes: 3c4287f6 ("nf_tables: Add set type for arbitrary concatenation of ranges")
Reported-by: etkaar <lists.netfilter.org@prvy.eu>
Signed-off-by: Florian Westphal <fw@strlen.de>
Reviewed-by: Stefano Brivio <sbrivio@redhat.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>