xfrm: delay initialization of offload path till its actually requested

XFRM offload path is probed even if offload isn't needed at all. Let's
make sure that x->type_offload pointer stays NULL for such path to
reduce ambiguity.

Fixes: 9d389d7f ("xfrm: Add a xfrm type offload.")
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>