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>

include/linux/sched.h

@@ -613,7 +613,7 @@ union rcu_special {
 		u8			blocked;
 		u8			need_qs;
 		u8			exp_hint; /* Hint for performance. */
-		u8			pad; /* No garbage from compiler! */
+		u8			need_mb; /* Readers need smp_mb(). */
 	} b; /* Bits. */
 	u32 s; /* Set of bits. */
 };
@@ -727,7 +727,7 @@ struct task_struct {
 #ifdef CONFIG_TASKS_TRACE_RCU
 	int				trc_reader_nesting;
 	int				trc_ipi_to_cpu;
-	bool				trc_reader_need_end;
+	union rcu_special		trc_reader_special;
 	bool				trc_reader_checked;
 	struct list_head		trc_holdout_list;
 #endif /* #ifdef CONFIG_TASKS_TRACE_RCU */