xfrm: fix error flow in case of add state fails

If add state fails in case of device offload, netdev refcount
will be negative since gc task is attempting to dev_free this state.
This is fixed by putting NULL in state dev field.

Signed-off-by: Aviad Yehezkel <aviadye@mellanox.com>
Signed-off-by: Boris Pismeny <borisp@mellanox.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>