netfilter: nf_tables: add nft_trans_commit_list_add_elem helper

netfilter: nf_tables: add nft_trans_commit_list_add_elem helper

Add and use a wrapper to append trans_elem structures to the
transaction log.

Unlike the existing helper, pass a gfp_t to indicate if sleeping
is allowed.

This will be used by a followup patch to realloc nft_trans_elem
structures after they gain a flexible array member to reduce
number of such container structures on the transaction list.

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