vhost/vsock: cleanup removing `len` variable

We can increment `total_len` directly and remove `len` since it
is no longer used for vhost_add_used().

Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Link: https://lore.kernel.org/r/20211122163525.294024-3-sgarzare@redhat.com


Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>