rcu-tasks: Remove preemption disablement around srcu_read_[un]lock() calls

Ever since the following commit:

	5a41344a ("srcu: Simplify __srcu_read_unlock() via this_cpu_dec()")

SRCU doesn't rely anymore on preemption to be disabled in order to
modify the per-CPU counter. And even then it used to be done from the API
itself.

Therefore and after checking further, it appears to be safe to remove
the preemption disablement around __srcu_read_[un]lock() in
exit_tasks_rcu_start() and exit_tasks_rcu_finish()
Suggested-by: Boqun Feng <boqun.feng@gmail.com>
Suggested-by: Paul E. McKenney <paulmck@kernel.org>
Suggested-by: Neeraj Upadhyay <quic_neeraju@quicinc.com>
Cc: Lai Jiangshan <jiangshanlai@gmail.com>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

kernel/rcu/tasks.h

@@ -1008,9 +1008,7 @@ EXPORT_SYMBOL_GPL(show_rcu_tasks_classic_gp_kthread);
  */
 void exit_tasks_rcu_start(void) __acquires(&tasks_rcu_exit_srcu)
 {
-	preempt_disable();
 	current->rcu_tasks_idx = __srcu_read_lock(&tasks_rcu_exit_srcu);
-	preempt_enable();
 }
 
 /*
@@ -1022,9 +1020,7 @@ void exit_tasks_rcu_finish(void) __releases(&tasks_rcu_exit_srcu)
 {
 	struct task_struct *t = current;
 
-	preempt_disable();
 	__srcu_read_unlock(&tasks_rcu_exit_srcu, t->rcu_tasks_idx);
-	preempt_enable();
 	exit_tasks_rcu_finish_trace(t);
 }