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>
diff --git a/net/key/af_key.c b/net/key/af_key.c
index 3440a46..5b22e01 100644
--- a/net/key/af_key.c
+++ b/net/key/af_key.c
@@ -3188,6 +3188,7 @@
 	return xp;
 
 out:
+	xp->walk.dead = 1;
 	xfrm_policy_destroy(xp);
 	return NULL;
 }