netfilter: nft_set_pipapo_avx2: split lookup function in two parts

Split the main avx2 lookup function into a helper.

This is a preparation patch: followup change will use the new helper
from the insertion path if possible.  This greatly improves insertion
performance when avx2 is supported.

Reviewed-by: Stefano Brivio <sbrivio@redhat.com>
Signed-off-by: Florian Westphal <fw@strlen.de>