This allows BTRFS encryption using zstd. Using zstd can increase
performance.

This change increases size of btrfs-progs only by 4K. The real effect on
size is only size of libzstd which is around 500K. This should be fine
on system requiring BTRFS so no variant without zstd was added.

Signed-off-by: Karel Kočí <karel.koci@nic.cz>