xfs: add missing assert in xfs_fsmap_owner_from_rmap

The fsmap handler shouldn't fail silently if the rmap code ever feeds it
a special owner number that isn't known to the fsmap handler.
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>

fs/xfs/xfs_fsmap.c

@@ -146,6 +146,7 @@ xfs_fsmap_owner_from_rmap(
 		dest->fmr_owner = XFS_FMR_OWN_FREE;
 		break;
 	default:
+		ASSERT(0);
 		return -EFSCORRUPTED;
 	}
 	return 0;