xfs: Fix xfs_flush_unmap_range() range for RT

Currently xfs_flush_unmap_range() does unmap for a full RT extent range,
which we also want to ensure is clean and idle.

This code change is originally from Dave Chinner.

Reviewed-by: Christoph Hellwig <hch@lst.de&gt;4>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: John Garry <john.g.garry@oracle.com>
Signed-off-by: Chandan Babu R <chandanbabu@kernel.org>