xfs: Fix xfs_prepare_shift() range for RT

The RT extent range must be considered in the xfs_flush_unmap_range() call
to stabilize the boundary.

This code change is originally from Dave Chinner.

Reviewed-by: Christoph Hellwig <hch@lst.de>
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>