rcu: Move early-boot callbacks to no-CBs lists for no-CBs CPUs

When a CPU is first determined to be a no-CBs CPUs, this commit causes
any early boot callbacks to be moved to the no-CBs callback list,
allowing them to be invoked.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 92fd3ea..0317bf7 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2851,6 +2851,7 @@
 		 * and then drop through to queue the callback.
 		 */
 		BUG_ON(cpu != -1);
+		WARN_ON_ONCE(!rcu_is_watching());
 		if (!likely(rdp->nxtlist))
 			init_default_callback_list(rdp);
 	}