af_key: mark policy as dead before destroying

xfrm_policy_destroy() will oops if not dead policy is passed to it.
On error path in pfkey_compile_policy() exactly this happens.

Oopsable for CAP_NET_ADMIN owners.

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>