netfilter: nf_tables: disallow jump to implicit chain from set element

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: d0e2c7de

 ("netfilter: nf_tables: add NFT_CHAIN_BINDING")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>