btrfs: add readonly flag

setting the readonly flag prevents writes in case an error is detected

Signed-off-by: Arne Jansen <sensille@gmx.net>
diff --git a/fs/btrfs/ioctl.h b/fs/btrfs/ioctl.h
index 1a638ce..e5e0ee2 100644
--- a/fs/btrfs/ioctl.h
+++ b/fs/btrfs/ioctl.h
@@ -81,6 +81,7 @@
 					 * Intermittent error. */
 };
 
+#define BTRFS_SCRUB_READONLY	1
 struct btrfs_ioctl_scrub_args {
 	__u64 devid;				/* in */
 	__u64 start;				/* in */