rcu-tasks: Fix the comments for tasks_rcu_exit_srcu_stall_timer

The synchronize_srcu() has been removed by commit("rcu-tasks: Eliminate
deadlocks involving do_exit() and RCU tasks") in rcu_tasks_postscan.
This commit therefore fixes the tasks_rcu_exit_srcu_stall_timer comment.

Signed-off-by: Zqiang <qiang.zhang1211@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>