netfilter: nf_tables_offload: move indirect flow_block callback logic to core

Add nft_offload_init() and nft_offload_exit() function to deal with the
init and the exit path of the offload infrastructure.

Rename nft_indr_block_get_and_ing_cmd() to nft_indr_block_cb().

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