xfrm: change the type of xfrm_register_km and xfrm_unregister_km

Functions xfrm_register_km and xfrm_unregister_km do always return 0,
change the type of functions to void.

Signed-off-by: Zhengchao Shao <shaozhengchao@huawei.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>