xfs: only run scrub in dry run mode

When checking a filesystem, explicitly run xfs_scrub in dry run mode
so that it will not ever try to preen, fix, or optimize anything.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>

common/xfs

@@ -331,7 +331,7 @@ _check_xfs_filesystem()
 
 	if [ "$type" = "xfs" ]; then
 		if [ -n "$TEST_XFS_SCRUB" ] && [ -x "$XFS_SCRUB_PROG" ]; then
-			"$XFS_SCRUB_PROG" $scrubflag -vd $device >>$seqres.full
+			"$XFS_SCRUB_PROG" $scrubflag -v -d -n $device >>$seqres.full
 			if [ $? -ne 0 ]; then
 				_log_err "filesystem on $device failed scrub"
 				ok=0