netfilter: nft_set_pipapo: add helper to release pcpu scratch area

[ Upstream commit 47b1c03c ]

After next patch simple kfree() is not enough anymore, so add
a helper for it.

Reviewed-by: Stefano Brivio <sbrivio@redhat.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Stable-dep-of: 5a8cdf6f ("netfilter: nft_set_pipapo: remove scratch_aligned pointer")
Signed-off-by: Sasha Levin <sashal@kernel.org>