MEDIUM: mux_fcgi: Make sure we return the tasklet from fcgi_deferred_shut

In fcgi_deferred_shut, return the tasklet to tell the scheduler the
tasklet is still alive, it is not yet needed, but will be soon.

src/mux_fcgi.c

@@ -3940,7 +3940,7 @@ struct task *fcgi_deferred_shut(struct task *t, void *ctx, unsigned int state)
 	}
  end:
 	TRACE_LEAVE(FCGI_EV_STRM_SHUT);
-	return NULL;
+	return t;
 }
 
 static void fcgi_shut(struct stconn *sc, unsigned int mode, struct se_abort_info *reason)