rcu/nocb: Assert no callbacks while nocb kthread allocation fails

When a NOCB CPU fails to create a nocb kthread on bringup, the CPU is
then deoffloaded. The barrier mutex is locked at this stage. It is
typically used to protect against concurrent (de-)offloading and/or
concurrent rcu_barrier() that would otherwise risk a nocb locking
imbalance. However:

* rcu_barrier() can't run concurrently if it's the boot CPU on early
  boot-up.

* rcu_barrier() can run concurrently if it's a secondary CPU but it is
  expected to see 0 callbacks on this target because it's the first
  time it boots.

* (de-)offloading can't happen concurrently with smp_init(), as
  rcutorture is initialized later, at least not before device_initcall(),
  and userspace isn't available yet.

* (de-)offloading can't happen concurrently with cpu_up(), courtesy of
  cpu_hotplug_lock.

But:

* The lazy shrinker might run concurrently with cpu_up(). It shouldn't
  try to grab the nocb_lock and risk an imbalance due to lazy_len
  supposed to be 0 but be extra cautious.

* Also be cautious against resume from hibernation potential subtleties.

So keep the locking and add some assertions and comments.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Reviewed-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Neeraj Upadhyay <neeraj.upadhyay@kernel.org>