btrfs/ctree.h: trivial fixup the comment for struct btrfs_dev_extent with the right fields' names

index 80b6486..a515e4e 100644
@@ -597,9 +597,9 @@ struct btrfs_extent_ref_v0 {
 } __attribute__ ((__packed__));
 
 
-/* dev extents record free space on individual devices.  The owner
+/* dev extents record free space on individual devices.  The chunk_tree
  * field points back to the chunk allocation mapping tree that allocated
- * the extent.  The chunk tree uuid field is a way to double check the owner
+ * the extent.  The chunk_tree_uuid field is a way to double check the owner
  */
 struct btrfs_dev_extent {
 	__le64 chunk_tree;