netfilter: nf_tables: add flowtable offload control plane

This patch adds the NFTA_FLOWTABLE_FLAGS attribute that allows users to
specify the NF_FLOWTABLE_HW_OFFLOAD flag. This patch also adds a new
setup interface for the flowtable type to perform the flowtable offload
block callback configuration.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>