[CFR][ZFS] merge Illumos rev. 13387 (refcompressratio property)

[Martin Matuska]

Add a "REFCOMPRESSRATIO" property, which is the compression ratio based
on data
referenced. For snapshots, this is the same as COMPRESSRATIO, but for
filesystems/volumes, the COMPRESSRATIO is based on the data "USED" (ie,
includes blocks in children, but not blocks shared with the origin).

This is needed to figure out how much space a filesystem would use if it
were not compressed (ignoring snapshots).

https://www.illumos.org/issues/1092
https://www.illumos.org/projects/illumos-gate/repository/revisions/13387

If there are no objections, I will commit this to -HEAD

-- 
Martin Matuska
FreeBSD committer
http://blog.vx.sk

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