btrfs-tree: add root refs and extent flags fields

Lazily filling in accessor methods for btrfs objects as needed by bees.

Signed-off-by: Zygo Blaxell <bees@furryterror.org>

include/crucible/btrfs-tree.h

@@ -64,11 +64,13 @@ namespace crucible {
 		/// @{ Extent items (EXTENT_ITEM)
 		uint64_t extent_begin() const;
 		uint64_t extent_end() const;
+		uint64_t extent_flags() const;
 		uint64_t extent_generation() const;
 		/// @}
 
 		/// @{ Root items
 		uint64_t root_flags() const;
+		uint64_t root_refs() const;
 		/// @}
 
 		/// @{ Root backref items.