btrfs: scrub: update message regarding read-only status

Based on user feedback update the message printed when scrub fails to
start due to write requirements. To make a distinction add a device id
to the messages.
Reviewed-by: NJosef Bacik <josef@toxicpanda.com>
Signed-off-by: NDavid Sterba <dsterba@suse.com>

fs/btrfs/scrub.c

@@ -3866,8 +3866,9 @@ int btrfs_scrub_dev(struct btrfs_fs_info *fs_info, u64 devid, u64 start,
 	if (!is_dev_replace && !readonly &&
 	    !test_bit(BTRFS_DEV_STATE_WRITEABLE, &dev->dev_state)) {
 		mutex_unlock(&fs_info->fs_devices->device_list_mutex);
-		btrfs_err_in_rcu(fs_info, "scrub: device %s is not writable",
-				rcu_str_deref(dev->name));
+		btrfs_err_in_rcu(fs_info,
+			"scrub on devid %llu: filesystem on %s is not writable",
+				 devid, rcu_str_deref(dev->name));
 		ret = -EROFS;
 		goto out;
 	}