vsock/loopback: support MSG_ZEROCOPY for transport

Add 'msgzerocopy_allow()' callback for loopback transport.
Signed-off-by: Arseniy Krasnov <avkrasnov@salutedevices.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/vmw_vsock/vsock_loopback.c

@@ -47,6 +47,10 @@ static int vsock_loopback_cancel_pkt(struct vsock_sock *vsk)
 }
 
 static bool vsock_loopback_seqpacket_allow(u32 remote_cid);
+static bool vsock_loopback_msgzerocopy_allow(void)
+{
+	return true;
+}
 
 static struct virtio_transport loopback_transport = {
 	.transport = {
@@ -79,6 +83,8 @@ static struct virtio_transport loopback_transport = {
 		.seqpacket_allow          = vsock_loopback_seqpacket_allow,
 		.seqpacket_has_data       = virtio_transport_seqpacket_has_data,
 
+		.msgzerocopy_allow        = vsock_loopback_msgzerocopy_allow,
+
 		.notify_poll_in           = virtio_transport_notify_poll_in,
 		.notify_poll_out          = virtio_transport_notify_poll_out,
 		.notify_recv_init         = virtio_transport_notify_recv_init,