netfilter: nf_tables: make struct nft_trans first member of derived subtypes

There is 'struct nft_trans', the basic structure for all transactional
objects, and the the various different transactional objects, such as
nft_trans_table, chain, set, set_elem and so on.

Right now 'struct nft_trans' uses a flexible member at the tail
(data[]), and casting is needed to access the actual type-specific
members.

Change this to make the hierarchy visible in source code, i.e. make
struct nft_trans the first member of all derived subtypes.

This has several advantages:
1. pahole output reflects the real size needed by the particular subtype
2. allows to use container_of() to convert the base type to the actual
   object type instead of casting ->data to the overlay structure.
3. It makes it easy to add intermediate types.

'struct nft_trans' contains a 'binding_list' that is only needed
by two subtypes, so it should be part of the two subtypes, not in
the base structure.

But that makes it hard to interate over the binding_list, because
there is no common base structure.

A follow patch moves the bind list to a new struct:

 struct nft_trans_binding {
   struct nft_trans nft_trans;
   struct list_head binding_list;
 };

... and makes that structure the new 'first member' for both
nft_trans_chain and nft_trans_set.

No functional change intended in this patch.

Some numbers:
 struct nft_trans { /* size: 88, cachelines: 2, members: 5 */
 struct nft_trans_chain { /* size: 152, cachelines: 3, members: 10 */
 struct nft_trans_elem { /* size: 112, cachelines: 2, members: 4 */
 struct nft_trans_flowtable { /* size: 128, cachelines: 2, members: 5 */
 struct nft_trans_obj { /* size: 112, cachelines: 2, members: 4 */
 struct nft_trans_rule { /* size: 112, cachelines: 2, members: 5 */
 struct nft_trans_set { /* size: 120, cachelines: 2, members: 8 */
 struct nft_trans_table { /* size: 96, cachelines: 2, members: 2 */

Of particular interest is nft_trans_elem, which needs to be allocated
once for each pending (to be added or removed) set element.

Add BUILD_BUG_ON to check struct nft_trans is placed at the top of
the container structure.

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