rcu-tasks: Split ->trc_reader_need_end

This commit splits ->trc_reader_need_end by using the rcu_special union.
This change permits readers to check to see if a memory barrier is
required without any added overhead in the common case where no such
barrier is required.  This commit also adds the read-side checking.
Later commits will add the machinery to properly set the new
->trc_reader_special.b.need_mb field.

This commit also makes rcu_read_unlock_trace_special() tolerate nested
read-side critical sections within interrupt and NMI handlers.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

init/init_task.c

@@ -143,6 +143,7 @@ struct task_struct init_task
 #endif
 #ifdef CONFIG_TASKS_TRACE_RCU
 	.trc_reader_nesting = 0,
+	.trc_reader_special.s = 0,
 	.trc_holdout_list = LIST_HEAD_INIT(init_task.trc_holdout_list),
 #endif
 #ifdef CONFIG_CPUSETS