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 <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/key/af_key.c

···
 	return xp;
 
 out:
+	xp->walk.dead = 1;
 	xfrm_policy_destroy(xp);
 	return NULL;
 }