Revert "rcu-tasks: Fix synchronize_rcu_tasks() VS zap_pid_ns_processes()"

This reverts commit 28319d6dc5. The race
it fixed was subject to conditions that don't exist anymore since:

	1612160b91 ("rcu-tasks: Eliminate deadlocks involving do_exit() and RCU tasks")

This latter commit removes the use of SRCU that used to cover the
RCU-tasks blind spot on exit between the tasklist's removal and the
final preemption disabling. The task is now placed instead into a
temporary list inside which voluntary sleeps are accounted as RCU-tasks
quiescent states. This would disarm the deadlock initially reported
against PID namespace exit.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Reviewed-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

include/linux/rcupdate.h

@@ -209,7 +209,6 @@ void synchronize_rcu_tasks_rude(void);
 
 #define rcu_note_voluntary_context_switch(t) rcu_tasks_qs(t, false)
 void exit_tasks_rcu_start(void);
-void exit_tasks_rcu_stop(void);
 void exit_tasks_rcu_finish(void);
 #else /* #ifdef CONFIG_TASKS_RCU_GENERIC */
 #define rcu_tasks_classic_qs(t, preempt) do { } while (0)
@@ -218,7 +217,6 @@ void exit_tasks_rcu_finish(void);
 #define call_rcu_tasks call_rcu
 #define synchronize_rcu_tasks synchronize_rcu
 static inline void exit_tasks_rcu_start(void) { }
-static inline void exit_tasks_rcu_stop(void) { }
 static inline void exit_tasks_rcu_finish(void) { }
 #endif /* #else #ifdef CONFIG_TASKS_RCU_GENERIC */