netfilter: cleanup struct nft_table

Add comments for nlpid, family, udlen and udata in struct nft_table, and
afinfo is no longer a member of struct nft_table, so remove the comment
for it.

Signed-off-by: George Guo <guodongtai@kylinos.cn>
Signed-off-by: Florian Westphal <fw@strlen.de>