netfilter: nf_tables: prefer nft_trans_elem_alloc helper

Reduce references to sizeof(struct nft_trans_elem).
Preparation patch to move this to a flexiable array to store
elem references.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>