netfilter: nft_set_pipapo: constify lookup fn args where possible

[ Upstream commit f04df573 ]

Those get called from packet path, content must not be modified.
No functional changes intended.

Reviewed-by: Stefano Brivio <sbrivio@redhat.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Stable-dep-of: 791a615b ("netfilter: nf_set_pipapo: fix initial map fill")
Signed-off-by: Sasha Levin <sashal@kernel.org>