refperf: Remove redundant kfree() after torture_stop_kthread()

Remove unnecessary kfree(main_task) call in ref_scale_cleanup() as
torture_stop_kthread() already handles the memory cleanup for the
task structure internally.

The additional kfree(main_task) call after torture_stop_kthread()
is redundant and confusing since torture_stop_kthread() sets the
pointer to NULL, making this a no-op.

This pattern is consistent with other torture test modules where
torture_stop_kthread() is called without explicit kfree() of the
task pointer, as the torture framework manages the task lifecycle
internally.

Signed-off-by: Kaushlendra Kumar <kaushlendra.kumar@intel.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

kernel/rcu/refscale.c

@@ -1135,7 +1135,6 @@ ref_scale_cleanup(void)
 	kfree(reader_tasks);
 
 	torture_stop_kthread("main_task", main_task);
-	kfree(main_task);
 
 	// Do scale-type-specific cleanup operations.
 	if (cur_ops->cleanup != NULL)