xfrm: Migrate offload configuration

Add hardware offload configuration to XFRM_MSG_MIGRATE
using an option netlink attribute XFRMA_OFFLOAD_DEV.

In the existing xfrm_state_migrate(), the xfrm_init_state()
is called assuming no hardware offload by default. Even the
original xfrm_state is configured with offload, the setting will
be reset. If the device is configured with hardware offload,
it's reasonable to allow the device to maintain its hardware
offload mode. But the device will end up with offload disabled
after receiving a migration event when the device migrates the
connection from one netdev to another one.

The devices that support migration may work with different
underlying networks, such as mobile devices. The hardware setting
should be forwarded to the different netdev based on the
migration configuration. This change provides the capability
for user space to migrate from one netdev to another.

Test: Tested with kernel test in the Android tree located
      in https://android.googlesource.com/kernel/tests/


      The xfrm_tunnel_test.py under the tests folder in
      particular.
Signed-off-by: Chiachang Wang <chiachangwang@google.com>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>