net/vmw_vsock: fix NULL pointer dereference

In vsock_stream_connect(), a thread will enter schedule_timeout().
While being scheduled out, another thread can enter vsock_stream_connect()
as well and set vsk->transport to NULL. In case a signal was sent, the
first thread can leave schedule_timeout() and vsock_transport_cancel_pkt()
will be called right after. Inside vsock_transport_cancel_pkt(), a null
dereference will happen on transport->cancel_pkt.

Fixes: c0cfa2d8 ("vsock: add multi-transports support")
Signed-off-by: Norbert Slusarek <nslusarek@gmx.net>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Link: https://lore.kernel.org/r/trinity-c2d6cede-bfb1-44e2-85af-1fbc7f541715-1612535117028@3c-app-gmx-bap12Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/vmw_vsock/af_vsock.c

@@ -1233,7 +1233,7 @@ static int vsock_transport_cancel_pkt(struct vsock_sock *vsk)
 {
 	const struct vsock_transport *transport = vsk->transport;
 
-	if (!transport->cancel_pkt)
+	if (!transport || !transport->cancel_pkt)
 		return -EOPNOTSUPP;
 
 	return transport->cancel_pkt(vsk);