rcu: Add diagnostics to grace-period cleanup

At grace-period initialization time, RCU checks that all quiescent
states were really reported for the previous grace period.  Now that
grace-period cleanup has been split out of grace-period initialization,
this commit also performs those checks at grace-period cleanup time.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

kernel/rcu/tree.c

@@ -1920,6 +1920,8 @@ static void rcu_gp_cleanup(struct rcu_state *rsp)
 	rcu_for_each_node_breadth_first(rsp, rnp) {
 		raw_spin_lock_irq(&rnp->lock);
 		smp_mb__after_unlock_lock();
+		WARN_ON_ONCE(rcu_preempt_blocked_readers_cgp(rnp));
+		WARN_ON_ONCE(rnp->qsmask);
 		ACCESS_ONCE(rnp->completed) = rsp->gpnum;
 		rdp = this_cpu_ptr(rsp->rda);
 		if (rnp == rdp->mynode)