hv_sock: disable SO_RCVLOWAT support

For Hyper-V it is quiet difficult to support this socket option,due to
transport internals, so disable it.
Signed-off-by: NArseniy Krasnov <AVKrasnov@sberdevices.ru>
Reviewed-by: NDexuan Cui <decui@microsoft.com>
Reviewed-by: NStefano Garzarella <sgarzare@redhat.com>
Signed-off-by: NPaolo Abeni <pabeni@redhat.com>

net/vmw_vsock/hyperv_transport.c

@@ -815,6 +815,12 @@ int hvs_notify_send_post_enqueue(struct vsock_sock *vsk, ssize_t written,
 	return 0;
 }
 
+static
+int hvs_set_rcvlowat(struct vsock_sock *vsk, int val)
+{
+	return -EOPNOTSUPP;
+}
+
 static struct vsock_transport hvs_transport = {
 	.module                   = THIS_MODULE,
 
@@ -850,6 +856,7 @@ static struct vsock_transport hvs_transport = {
 	.notify_send_pre_enqueue  = hvs_notify_send_pre_enqueue,
 	.notify_send_post_enqueue = hvs_notify_send_post_enqueue,
 
+	.set_rcvlowat             = hvs_set_rcvlowat
 };
 
 static bool hvs_check_transport(struct vsock_sock *vsk)