netfilter: nf_tables: cleanup documentation

netfilter: nf_tables: cleanup documentation

- Correct comments for nlpid, family, udlen and udata in struct nft_table,
  and afinfo is no longer a member of enum nft_set_class.

- Add comment for data in struct nft_set_elem.

- Add comment for flags in struct nft_ctx.

- Add comments for timeout in struct nft_set_iter, and flags is not a
  member of struct nft_set_iter, remove the comment for it.

- Add comments for commit, abort, estimate and gc_init in struct
  nft_set_ops.

- Add comments for pending_update, num_exprs, exprs and catchall_list
  in struct nft_set.

- Add comment for ext_len in struct nft_set_ext_tmpl.

- Add comment for inner_ops in struct nft_expr_type.

- Add comments for clone, destroy_clone, reduce, gc, offload,
  offload_action, offload_stats in struct nft_expr_ops.

- Add comments for blob_gen_0, blob_gen_1, bound, genmask, udlen, udata,
  blob_next in struct nft_chain.

- Add comment for flags in struct nft_base_chain.

- Add comments for udlen, udata in struct nft_object.

- Add comment for type in struct nft_object_ops.

- Add comment for hook_list in struct nft_flowtable, and remove comments
  for dev_name and ops which are not members of struct nft_flowtable.

Signed-off-by: George Guo <guodongtai@kylinos.cn>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>