btrfs: dev-replace: set result code of cancel by status of scrub

The device-replace needs to check the result code of the scrub workers
in btrfs_dev_replace_cancel and distinguish if successful cancel
operation and when the there was no operation running.

If btrfs_scrub_cancel() fails, return
BTRFS_IOCTL_DEV_REPLACE_RESULT_NOT_STARTED so that user can try
to cancel the replace again.
Signed-off-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: David Sterba <dsterba@suse.com>
[ update changelog ]
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/dev-replace.c

@@ -805,16 +805,23 @@ int btrfs_dev_replace_cancel(struct btrfs_fs_info *fs_info)
 		btrfs_dev_replace_write_unlock(dev_replace);
 		break;
 	case BTRFS_IOCTL_DEV_REPLACE_STATE_STARTED:
-		result = BTRFS_IOCTL_DEV_REPLACE_RESULT_NO_ERROR;
 		tgt_device = dev_replace->tgtdev;
 		src_device = dev_replace->srcdev;
 		btrfs_dev_replace_write_unlock(dev_replace);
-		btrfs_scrub_cancel(fs_info);
-		/* btrfs_dev_replace_finishing() will handle the cleanup part */
+		ret = btrfs_scrub_cancel(fs_info);
+		if (ret < 0) {
+			result = BTRFS_IOCTL_DEV_REPLACE_RESULT_NOT_STARTED;
+		} else {
+			result = BTRFS_IOCTL_DEV_REPLACE_RESULT_NO_ERROR;
+			/*
+			 * btrfs_dev_replace_finishing() will handle the
+			 * cleanup part
+			 */
 			btrfs_info_in_rcu(fs_info,
 				"dev_replace from %s (devid %llu) to %s canceled",
 				btrfs_dev_name(src_device), src_device->devid,
 				btrfs_dev_name(tgt_device));
+		}
 		break;
 	case BTRFS_IOCTL_DEV_REPLACE_STATE_SUSPENDED:
 		/*