netfilter: nf_tables: increase maximum devices number per flowtable

Rise the maximum limit of devices per flowtable up to 256. Rename
NFT_FLOWTABLE_DEVICE_MAX to NFT_NETDEVICE_MAX in preparation to reuse
the netdev hook parser for ingress basechain.

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