netfilter: nft_set_pipapo: merge pipapo_get/lookup

The matching algorithm has implemented thrice:
1. data path lookup, generic version
2. data path lookup, avx2 version
3. control plane lookup

Merge 1 and 3 by refactoring pipapo_get as a common helper, then make
nft_pipapo_lookup and nft_pipapo_get both call the common helper.

Aside from the code savings this has the benefit that we no longer allocate
temporary scratch maps for each control plane get and insertion operation.

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