xfs: check reference counts of gaps between rt refcount records

xfs: check reference counts of gaps between rt refcount records

If there's a gap between records in the rt refcount btree, we ought to
cross-reference the gap with the rtrmap records to make sure that there
aren't any overlapping records for a region that doesn't have any shared
ownership.

Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>