rcu: Print human-readable message for schedule() in RCU reader

The WARN_ON_ONCE() invocation within the CONFIG_PREEMPT=y version of
rcu_note_context_switch() triggers when there is a voluntary context
switch in an RCU read-side critical section, but there is quite a gap
between the output of that WARN_ON_ONCE() and this RCU-usage error.
This commit therefore converts the WARN_ON_ONCE() to a WARN_ONCE()
that explicitly describes the problem in its message.
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

kernel/rcu/tree_plugin.h

@@ -346,7 +346,7 @@ void rcu_note_context_switch(bool preempt)
 
 	trace_rcu_utilization(TPS("Start context switch"));
 	lockdep_assert_irqs_disabled();
-	WARN_ON_ONCE(!preempt && rcu_preempt_depth() > 0);
+	WARN_ONCE(!preempt && rcu_preempt_depth() > 0, "Voluntary context switch within RCU read-side critical section!");
 	if (rcu_preempt_depth() > 0 &&
 	    !t->rcu_read_unlock_special.b.blocked) {