NBD: Write Zeroes and Rotational

The NBD protocol has grown a number of new features over the years. Unfortunately, some of those features are not (yet?) supported by the Linux kernel. I suggested a few times over the years that the maintainer of the NBD driver in the kernel, Josef Bacik, take a look at these features, but he hasn't done so; presumably he has other priorities. As with anything in the open source world, if you…