This patch adds ZSTD compression to ZFS
It currently supports 3 (of the 22)the default 19 compression levels, 1 (min), 3 (default), and 19 (max,. 20-22 are 'ultra')Support for a subset of the negative levels is planned for a future commit.
It includes the required changes to boot from zstd compressed datasets for BIOS/LEGACY and UEFI.