xfrm: add packet offload mode to xfrm state

xfrm: add packet offload mode to xfrm state

Allow users to configure xfrm states with packet offload type.

Packet offload mode:
  ip xfrm state offload packet dev <if-name> dir <in|out>
Crypto offload mode:
  ip xfrm state offload crypto dev <if-name> dir <in|out>
  ip xfrm state offload dev <if-name> dir <in|out>

The latter variant configures crypto offload mode and is needed
to provide backward compatibility.

Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: David Ahern <dsahern@kernel.org>