netfilter: nft_set_pipapo: move prove_locking helper around

Preparation patch, the helper will soon get called from insert
function too.

Signed-off-by: Florian Westphal <fw@strlen.de>
Reviewed-by: Stefano Brivio <sbrivio@redhat.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>