zstd patch still needed in 5.9 to use zstd compressed modules

[veganvelociraptor]

It seems the zstd addition to 5.9 allowing a zstd compressed kernel imaged, omitted the parts of the patch allowing the modules to be zstd compressed.

I changed zstd.mypatch from 5.8 and retained the bits and pieces needed to be able to compress the modules as well: https://pastebin.com/XHKuWR2g

It's a quick and dirty hack, I'm not sure the changes to xxhash.c are actually needed, nor have I tried out if it works with 5.10 (though it should).