compiler: Fixup nbytes-avail-mapper

Author: FabioLuporini

devito/operator/operator.py

@@ -1304,7 +1304,11 @@ def nbytes_avail_mapper(self):
                     continue
                 try:
                     if i._mem_mapped:
-                        mapper[device_layer] -= i.nbytes
+                        try:
+                            v = self[i.name]._obj.nbytes
+                        except AttributeError:
+                            v = i.nbytes
+                        mapper[device_layer] -= v
                 except AttributeError:
                     pass