Fix raw disk space usage for chunks

The calculation did not make any sense for profiles like RAID0, RAID10,
RAID5 and RAID6. Use the new helper to calculate the size of a device
extent that is in use on disks to back the storage for this chunk.

heatmap.py

@@ -367,7 +367,7 @@ def walk_chunks(fs, devices=None, order=None, size=None,
         except btrfs.ctree.ItemNotFoundError:
             continue
         used_pct = block_group.used / block_group.length
-        length = chunk.length * len(stripes)
+        length = btrfs.utils.chunk_to_dev_extent_length(chunk) * len(stripes)
         if verbose >= 1:
             print(block_group)
             print(chunk)