af_key: Fix memory leak in key_notify_policy.

We leak the allocated out_skb in case
pfkey_xfrm_policy2msg() fails. Fix this
by freeing it on error.
Reported-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>

net/key/af_key.c

@@ -2202,8 +2202,10 @@ static int key_notify_policy(struct xfrm_policy *xp, int dir, const struct km_ev
 		return PTR_ERR(out_skb);
 
 	err = pfkey_xfrm_policy2msg(out_skb, xp, dir);
-	if (err < 0)
+	if (err < 0) {
+		kfree_skb(out_skb);
 		return err;
+	}
 
 	out_hdr = (struct sadb_msg *) out_skb->data;
 	out_hdr->sadb_msg_version = PF_KEY_V2;