rcu: Add preemptibility checks in rcu_sched_qs() and rcu_bh_qs()

This commit adds WARN_ON_ONCE() calls that trigger if either
rcu_sched_qs() or rcu_bh_qs() are invoked with preemption enabled.
In the immortal words of Peter Zijlstra: "these are much harder to ignore
than comments".

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

kernel/rcu/tree.c

@@ -250,6 +250,7 @@ static int rcu_gp_in_progress(struct rcu_state *rsp)
  */
 void rcu_sched_qs(void)
 {
+	RCU_LOCKDEP_WARN(preemptible(), "rcu_sched_qs() invoked with preemption enabled!!!");
 	if (!__this_cpu_read(rcu_sched_data.cpu_no_qs.s))
 		return;
 	trace_rcu_grace_period(TPS("rcu_sched"),
@@ -265,6 +266,7 @@ void rcu_sched_qs(void)
 
 void rcu_bh_qs(void)
 {
+	RCU_LOCKDEP_WARN(preemptible(), "rcu_bh_qs() invoked with preemption enabled!!!");
 	if (__this_cpu_read(rcu_bh_data.cpu_no_qs.s)) {
 		trace_rcu_grace_period(TPS("rcu_bh"),
 				       __this_cpu_read(rcu_bh_data.gpnum),