vsock/loopback: support MSG_ZEROCOPY for transport

Add 'msgzerocopy_allow()' callback for loopback transport.

Signed-off-by: Arseniy Krasnov <[e-mail]>
Reviewed-by: Stefano Garzarella <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/vmw_vsock/vsock_loopback.c

···
 }
 
 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 = {
···
 		.seqpacket_enqueue        = virtio_transport_seqpacket_enqueue,
 		.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,