supporting zstd fast levels on Btrfs

[Chris Murphy <lists@xxxxxxxxxxxxxxxxx>]

Looks like since zstd v1.3.4 there are five negative levels (also
--fast levels), that looks like they'd be in the ballpark of competing
with lz4. That might be useful even with some of the faster NVMe
drives, ~2G/s.

Any idea if it's possible or even likely?

Thanks!

-- 
Chris Murphy