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>

fs/xfs/xfs_bmap_util.c

@@ -902,7 +902,7 @@ xfs_prepare_shift(
 	struct xfs_inode	*ip,
 	loff_t			offset)
 {
-	struct xfs_mount	*mp = ip->i_mount;
+	unsigned int		rounding;
 	int			error;
 
 	/*
@@ -920,11 +920,13 @@ xfs_prepare_shift(
 	 * with the full range of the operation. If we don't, a COW writeback
 	 * completion could race with an insert, front merge with the start
 	 * extent (after split) during the shift and corrupt the file. Start
-	 * with the block just prior to the start to stabilize the boundary.
+	 * with the allocation unit just prior to the start to stabilize the
+	 * boundary.
 	 */
-	offset = round_down(offset, mp->m_sb.sb_blocksize);
+	rounding = xfs_inode_alloc_unitsize(ip);
+	offset = rounddown_64(offset, rounding);
 	if (offset)
-		offset -= mp->m_sb.sb_blocksize;
+		offset -= rounding;
 
 	/*
 	 * Writeback and invalidate cache for the remainder of the file as we're