Include zstd compression format

[dxzdxz1]

There is a patch made for linux that adds support for zstd, it improves compression without compromise performance. Is much better than zlib but faster as lzo.

Here is the patch

I know that your driver has nothing to do with the linux code, so I don't know if it's possible to port support for zstd, but since it's a great alternative for transparent compression I believe it's worth to try it out.

[maharmstone]

I'm not going to start including features until they've been accepted in Linux, sorry - that'd be impossible to maintain. I'll have a look once it reaches the mainline kernel.