netfilter: nf_tables: map basechain priority to hardware priority

This patch adds initial support for offloading basechains using the
priority range from 1 to 65535. This is restricting the netfilter
priority range to 16-bit integer since this is what most drivers assume
so far from tc. It should be possible to extend this range of supported
priorities later on once drivers are updated to support for 32-bit
integer priorities.

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