xfrm: fix a read lock imbalance in make_blackhole

if xfrm_policy_get_afinfo returns 0, it has already released the read
lock, xfrm_policy_put_afinfo should not be called again.
Signed-off-by: NLi RongQing <roy.qing.li@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/xfrm/xfrm_policy.c

@@ -1764,7 +1764,7 @@ static struct dst_entry *make_blackhole(struct net *net, u16 family,
 
 	if (!afinfo) {
 		dst_release(dst_orig);
-		ret = ERR_PTR(-EINVAL);
+		return ERR_PTR(-EINVAL);
 	} else {
 		ret = afinfo->blackhole_route(net, dst_orig);
 	}