netfilter: nf_tables: add NFT_CHAIN_BINDING

This new chain flag specifies that:

* the kernel dynamically allocates the chain name, if no chain name
  is specified.

* If the immediate expression that refers to this chain is removed,
  then this bound chain (and its content) is destroyed.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>