src: support for offload chain flag

src: support for offload chain flag

This patch extends the basechain definition to allow users to specify
the offload flag. This flag enables hardware offload if your drivers
supports it.

 # cat file.nft
 table netdev x {
    chain y {
       type filter hook ingress device eth0 priority 10; flags offload;
    }
 }
 # nft -f file.nft

Note: You have to enable offload via ethtool:

 # ethtool -K eth0 hw-tc-offload on

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