RxRPC: do not unlock unheld spinlock in rxrpc_connect_exclusive()

If rx->conn is not NULL, rxrpc_connect_exclusive() does not
acquire the transport's client lock, but it still releases it.

The patch adds locking of the spinlock to this path.

Found by Linux Driver Verification project (linuxtesting.org).
Signed-off-by: NAlexey Khoroshilov <khoroshilov@ispras.ru>
Signed-off-by: NDavid Howells <dhowells@redhat.com>

net/rxrpc/ar-connection.c

@@ -381,6 +381,8 @@ static int rxrpc_connect_exclusive(struct rxrpc_sock *rx,
 
 		rxrpc_assign_connection_id(conn);
 		rx->conn = conn;
+	} else {
+		spin_lock(&trans->client_lock);
 	}
 
 	/* we've got a connection with a free channel and we can now attach the