rcu: Protect rcu_node accesses during CPU stall warnings

The print_other_cpu_stall() function accesses a number of rcu_node
fields without protection from the ->lock.  In theory, this is not
a problem because the fields accessed are all integers, but in
practice the compiler can get nasty.  Therefore, the commit extends
the existing critical section to cover the entire loop body.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

kernel/rcutree.c

@@ -746,15 +746,16 @@ static void print_other_cpu_stall(struct rcu_state *rsp)
 	rcu_for_each_leaf_node(rsp, rnp) {
 		raw_spin_lock_irqsave(&rnp->lock, flags);
 		ndetected += rcu_print_task_stall(rnp);
-		raw_spin_unlock_irqrestore(&rnp->lock, flags);
-		if (rnp->qsmask == 0)
-			continue;
+		if (rnp->qsmask != 0) {
 			for (cpu = 0; cpu <= rnp->grphi - rnp->grplo; cpu++)
 				if (rnp->qsmask & (1UL << cpu)) {
-				print_cpu_stall_info(rsp, rnp->grplo + cpu);
+					print_cpu_stall_info(rsp,
+							     rnp->grplo + cpu);
 					ndetected++;
 				}
 		}
+		raw_spin_unlock_irqrestore(&rnp->lock, flags);
+	}
 
 	/*
 	 * Now rat on any tasks that got kicked up to the root rcu_node