xfs/201: use min_dio_alignment size to replace 512b

This case use hard-code 512, but in 4k sector size device,
it will fail.

So I call _min_dio_alignment() to get the sector size, then
replace `512`.

Signed-off-by: Zorro Lang <zlang@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>

tests/xfs/201

@@ -32,15 +32,6 @@ here=`pwd`
 tmp=/tmp/$$
 status=1	# failure is the default!
 
-do_pwrite()
-{
-	offset=`expr $1 \* 512`
-	end=`expr $2 \* 512`
-	length=`expr $end - $offset`
-
-	xfs_io -d -f $file -c "pwrite $offset $length" >/dev/null
-}
-
 _cleanup()
 {
 	umount $SCRATCH_MNT
@@ -57,6 +48,17 @@ _supported_os Linux
 
 file=$SCRATCH_MNT/f
 
+min_align=`_min_dio_alignment $SCRATCH_DEV`
+
+do_pwrite()
+{
+	offset=`expr $1 \* $min_align`
+	end=`expr $2 \* $min_align`
+	length=`expr $end - $offset`
+
+	xfs_io -d -f $file -c "pwrite $offset $length" >/dev/null
+}
+
 _require_scratch
 
 _scratch_mkfs_xfs >/dev/null 2>&1