md: never update metadata when array is read-only.

Normally we don't even try to update the metadata if
the array is read-only.  However future patches
will increase the number of things that can happen on a read-only
array, so it is safest to explicitly disable this.

Every time that mddev->ro is set to 0, either
 - md_update_sb will be called again (at least if MD_CHANGE_DEVS
   is set) or
 - the mddev->thread is scheduled, which will also run
   md_update_sb if needed.

So this is safe: if the array ever become read-write the
metadata will be updated.
Signed-off-by: NeilBrown <neilb@suse.de>

drivers/md/md.c

@@ -2411,6 +2411,11 @@ static void md_update_sb(struct mddev * mddev, int force_change)
 	int nospares = 0;
 	int any_badblocks_changed = 0;
 
+	if (mddev->ro) {
+		if (force_change)
+			set_bit(MD_CHANGE_DEVS, &mddev->flags);
+		return;
+	}
 repeat:
 	/* First make sure individual recovery_offsets are correct */
 	rdev_for_each(rdev, mddev) {