netfilter: nf_tables: merge nft_rules_old structure and end of ruleblob marker

In order to free the rules in a chain via call_rcu, the rule array used
to stash a rcu_head and space for a pointer at the end of the rule array.

When the current nft_rule_dp blob format got added in
2c865a8a ("netfilter: nf_tables: add rule blob layout"), this results
in a double-trailer:

  size (unsigned long)
  struct nft_rule_dp
    struct nft_expr
         ...
    struct nft_rule_dp
     struct nft_expr
         ...
    struct nft_rule_dp (is_last=1) // Trailer

The trailer, struct nft_rule_dp (is_last=1), is not accounted for in size,
so it can be located via start_addr + size.

Because the rcu_head is stored after 'start+size' as well this means the
is_last trailer is *aliased* to the rcu_head (struct nft_rules_old).

This is harmless, because at this time the nft_do_chain function never
evaluates/accesses the trailer, it only checks the address boundary:

        for (; rule < last_rule; rule = nft_rule_next(rule)) {
...

But this way the last_rule address has to be stashed in the jump
structure to restore it after returning from a chain.

nft_do_chain stack usage has become way too big, so put it on a diet.

Without this patch is impossible to use
        for (; !rule->is_last; rule = nft_rule_next(rule)) {

... because on free, the needed update of the rcu_head will clobber the
nft_rule_dp is_last bit.

Furthermore, also stash the chain pointer in the trailer, this allows
to recover the original chain structure from nf_tables_trace infra
without a need to place them in the jump struct.

After this patch it is trivial to diet the jump stack structure,
done in the next two patches.

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