[btree_builder] Fix reference counts of btree nodes

A leaf node should be counted only if it is referenced by some internal
nodes, since the leaves generated by <def> sections might be unshifted
and merged with exclusive mappings or other shared defs, or they might
not even be used by any of the devices if the xml was tampered. The
internal nodes should be handled in the same manner.

The new 'reserve' operation is designed for this purpose. Applications
could reserve a block for later use, without setting its ref count
immediately. This way saves the hassles of tracking unused leaves.

src/thin/restore.rs

@@ -330,7 +330,6 @@ pub fn restore(opts: ThinRestoreOptions) -> Result<()> {
     // Build data space map
     let data_sm_root = build_data_sm(&mut w, pass.data_sm.lock().unwrap().deref())?;
 
-    // FIXME: I think we need to decrement the shared leaves
     // Build metadata space map
     let metadata_sm_root = build_metadata_sm(&mut w)?;