net: Fix one possible memleak in ip_setup_cork

It would allocate memory in this function when the cork->opt is NULL. But
the memory isn't freed if failed in the latter rt check, and return error
directly. It causes the memleak if its caller is ip_make_skb which also
doesn't free the cork->opt when meet a error.

Now move the rt check ahead to avoid the memleak.

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

net/ipv4/ip_output.c

···
 	struct ip_options_rcu *opt;
 	struct rtable *rt;
 
+	rt = *rtp;
+	if (unlikely(!rt))
+		return -EFAULT;
+
 	/*
 	 * setup for corking.
 	 */
···
 		cork->flags |= IPCORK_OPT;
 		cork->addr = ipc->addr;
 	}
-	rt = *rtp;
-	if (unlikely(!rt))
-		return -EFAULT;
+
 	/*
 	 * We steal reference to this route, caller should not release it
 	 */