xfrm: move xfrm_garbage_collect out of xfrm_policy_flush

Now we will force to do garbage collection if any policy removed in
xfrm_policy_flush(). But during xfrm_net_exit(). We call flow_cache_fini()
first and set set fc->percpu to NULL. Then after we call xfrm_policy_fini()
-> frxm_policy_flush() -> flow_cache_flush(), we will get NULL pointer
dereference when check percpu_empty. The code path looks like:

flow_cache_fini()
  - fc->percpu = NULL
xfrm_policy_fini()
  - xfrm_policy_flush()
    - xfrm_garbage_collect()
      - flow_cache_flush()
        - flow_cache_percpu_empty()
	  - fcp = per_cpu_ptr(fc->percpu, cpu)

To reproduce, just add ipsec in netns and then remove the netns.

v2:
As Xin Long suggested, since only two other places need to call it. move
xfrm_garbage_collect() outside xfrm_policy_flush().

v3:
Fix subject mismatch after v2 fix.

Fixes: 35db069121 ("xfrm: do the garbage collection after flushing policy")
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Reviewed-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>

net/key/af_key.c

@@ -2755,6 +2755,8 @@ static int pfkey_spdflush(struct sock *sk, struct sk_buff *skb, const struct sad
 	int err, err2;
 
 	err = xfrm_policy_flush(net, XFRM_POLICY_TYPE_MAIN, true);
+	if (!err)
+		xfrm_garbage_collect(net);
 	err2 = unicast_flush_resp(sk, hdr);
 	if (err || err2) {
 		if (err == -ESRCH) /* empty table - old silent behavior */

net/xfrm/xfrm_policy.c

@@ -1006,10 +1006,6 @@ int xfrm_policy_flush(struct net *net, u8 type, bool task_valid)
 		err = -ESRCH;
 out:
 	spin_unlock_bh(&net->xfrm.xfrm_policy_lock);
-
-	if (cnt)
-		xfrm_garbage_collect(net);
-
 	return err;
 }
 EXPORT_SYMBOL(xfrm_policy_flush);

net/xfrm/xfrm_user.c

@@ -2027,6 +2027,7 @@ static int xfrm_flush_policy(struct sk_buff *skb, struct nlmsghdr *nlh,
 			return 0;
 		return err;
 	}
+	xfrm_garbage_collect(net);
 
 	c.data.type = type;
 	c.event = nlh->nlmsg_type;