xfs: report refcount information to userspace

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.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
-----BEGIN PGP SIGNATURE-----
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=0QCR
-----END PGP SIGNATURE-----