sctp: sctp_transport_lookup_process should rcu_read_unlock when transport is null

Prior to this patch, sctp_transport_lookup_process didn't rcu_read_unlock
when it failed to find a transport by sctp_addrs_lookup_transport.

This patch is to fix it by moving up rcu_read_unlock right before checking
transport and also to remove the out path.

Fixes: 1cceda7849 ("sctp: fix the issue sctp_diag uses lock_sock in rcu_read_lock")
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Acked-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sctp/socket.c

@@ -4472,18 +4472,17 @@ int sctp_transport_lookup_process(int (*cb)(struct sctp_transport *, void *),
 				  const union sctp_addr *paddr, void *p)
 {
 	struct sctp_transport *transport;
-	int err = -ENOENT;
+	int err;
 
 	rcu_read_lock();
 	transport = sctp_addrs_lookup_transport(net, laddr, paddr);
-	if (!transport)
-		goto out;
-
 	rcu_read_unlock();
+	if (!transport)
+		return -ENOENT;
+
 	err = cb(transport, p);
 	sctp_transport_put(transport);
 
-out:
 	return err;
 }
 EXPORT_SYMBOL_GPL(sctp_transport_lookup_process);