storage/reflink: don't bother using _get_size() in create()

Only the nominal size is available at this point.

qubes/storage/reflink.py

@@ -156,7 +156,7 @@ class ReflinkVolume(qubes.storage.Volume):
     def create(self):
         self._remove_all_images()
         if self.save_on_stop and not self.snap_on_start:
-            _create_sparse_file(self._path_clean, self._get_size())
+            _create_sparse_file(self._path_clean, self._size)
         return self
 
     @_coroutinized