[1.7.x] Used the same instance of atomic for entry and exit.

Since all state is maintained on the connection at this time and none in
the atomic, it doesn't matter, but it could introduce some subtle bugs
if the implementation changed in the future.

Backport of 0aa4c6c3 from master

django/db/backends/schema.py

@@ -72,7 +72,8 @@ class BaseDatabaseSchemaEditor(object):
     def __enter__(self):
         self.deferred_sql = []
         if self.connection.features.can_rollback_ddl:
-            atomic(self.connection.alias).__enter__()
+            self.atomic = atomic(self.connection.alias)
+            self.atomic.__enter__()
         return self
 
     def __exit__(self, exc_type, exc_value, traceback):
@@ -80,7 +81,7 @@ class BaseDatabaseSchemaEditor(object):
             for sql in self.deferred_sql:
                 self.execute(sql)
         if self.connection.features.can_rollback_ddl:
-            atomic(self.connection.alias).__exit__(exc_type, exc_value, traceback)
+            self.atomic.__exit__(exc_type, exc_value, traceback)
 
     # Core utility functions