MFC r223623:

MFC r223623:
Add a new "REFCOMPRESSRATIO" property.

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).

Illumos-gate revision:	13387

Obtained from:	Illumos (Feature #1092)

git-svn-id: svn://svn.freebsd.org/base/stable/8@223950 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f