Extend struct nft_data_desc to add a flag field that specifies
nft_data_init() is being called for set element data.
Use it to disallow jump to implicit chain from set element, only jump
to chain via immediate expression is allowed.
Fixes: d0e2c7de92 ("netfilter: nf_tables: add NFT_CHAIN_BINDING")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>