netfilter: nft_set_pipapo: prepare pipapo_get helper for on-demand clone

The helper uses priv->clone unconditionally which will fail once we do
the clone conditionally on first insert or removal.

'nft get element' from userspace needs to use priv->match since this
runs from rcu read side lock section.

Prepare for this by passing the match backend data as argument.

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