netfilter: nft_set_pipapo: release elements in clone only from destroy path

Clone already always provides a current view of the lookup table, use it
to destroy the set, otherwise it is possible to destroy elements twice.

This fix requires:

 212ed75d ("netfilter: nf_tables: integrate pipapo into commit protocol")

which came after:

 9827a0e6 ("netfilter: nft_set_pipapo: release elements in clone from abort path").

Fixes: 9827a0e6 ("netfilter: nft_set_pipapo: release elements in clone from abort path")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>