netns: fix unbalanced spin_lock on error

Unlock was missing on error path.

Fixes: 95f38411df05 ("netns: use a spin_lock to protect nsid management")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/net_namespace.c

@@ -565,6 +565,7 @@ static int rtnl_net_newid(struct sk_buff *skb, struct nlmsghdr *nlh)
 
 	spin_lock_irqsave(&nsid_lock, flags);
 	if (__peernet2id(net, peer) >= 0) {
+		spin_unlock_irqrestore(&nsid_lock, flags);
 		err = -EEXIST;
 		goto out;
 	}