xfsprogs: report refcount information to userspace [57/62]

Create a new ioctl to report the number of owners of each disk block so
that reflink-aware defraggers can make better decisions about which
extents to target.

This has been running on the djcloud for months with no problems.  Enjoy!

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
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