netfilter: nft_set_pipapo: make pipapo_clone helper return NULL

Currently it returns an error pointer, but the only possible failure
is ENOMEM.

After a followup patch, we'd need to discard the errno code, i.e.

x = pipapo_clone()
if (IS_ERR(x))
	return NULL

or make more changes to fix up callers to expect IS_ERR() code
from set->ops->deactivate().

So simplify this and make it return ptr-or-null.

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