Update the free cluster count when unmounting.

Also decrement the free cluster count when allocating a cluster.

src/fat/volume.rs

@@ -1042,6 +1042,7 @@ impl FatVolume {
                 Err(e) => return Err(e),
             };
         debug!("Next free cluster is {:?}", self.next_free_cluster);
+        // Record that we've allocated a cluster
         if let Some(ref mut number_free_cluster) = self.free_clusters_count {
             *number_free_cluster -= 1;
         };

src/volume_mgr.rs

@@ -334,6 +334,7 @@ where
     /// You can't close it if there are any files or directories open on it.
     pub fn close_volume(&self, volume: RawVolume) -> Result<(), Error<D::Error>> {
         let mut data = self.data.try_borrow_mut().map_err(|_| Error::LockError)?;
+        let data = data.deref_mut();
 
         for f in data.open_files.iter() {
             if f.raw_volume == volume {
@@ -349,6 +350,12 @@ where
 
         let volume_idx = data.get_volume_by_id(volume)?;
 
+        match &mut data.open_volumes[volume_idx].volume_type {
+            VolumeType::Fat(fat) => {
+                fat.update_info_sector(&mut data.block_cache)?;
+            },
+        }
+
         data.open_volumes.swap_remove(volume_idx);
 
         Ok(())