netfilter: nf_tables: do not store nft_ctx in transaction objects

nft_ctx is huge and most of the information stored within isn't used
at all.

Remove nft_ctx member from the base transaction structure and store
only what is needed.

After this change, relevant struct sizes are:

struct nft_trans_chain { /* size: 120 (-32), cachelines: 2, members: 10 */
struct nft_trans_elem { /* size: 72 (-40), cachelines: 2, members: 4 */
struct nft_trans_flowtable { /* size: 80 (-48), cachelines: 2, members: 5 */
struct nft_trans_obj { /* size: 72 (-40), cachelines: 2, members: 4 */
struct nft_trans_rule { /* size: 80 (-32), cachelines: 2, members: 6 */
struct nft_trans_set { /* size: 96 (-24), cachelines: 2, members: 8 */
struct nft_trans_table { /* size: 56 (-40), cachelines: 1, members: 2 */

struct nft_trans_elem can now be allocated from kmalloc-96 instead of
kmalloc-128 slab.
A further reduction by 8 bytes would even allow for kmalloc-64.

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