rcu/nocb: (De-)offload callbacks on offline CPUs only

Currently callbacks can be (de-)offloaded only on online CPUs. This
involves an overly elaborated state machine in order to make sure that
callbacks are always handled during the process while ensuring
synchronization between rcu_core and NOCB kthreads.

The only potential user of NOCB (de-)offloading appears to be a
nohz_full toggling interface through cpusets. And the general agreement
is now to work toward toggling the nohz_full state on offline CPUs to
simplify the whole picture.

Therefore, convert the (de-)offloading to only support offline CPUs.
This involves the following changes:

* Call rcu_barrier() before deoffloading. An offline offloaded CPU may
  still carry callbacks in its queue ignored by
  rcutree_migrate_callbacks(). Those callbacks must all be flushed
  before switching to a regular queue because no more kthreads will
  handle those before the CPU ever gets re-onlined.

  This means that further calls to rcu_barrier() will find an empty
  queue until the CPU goes through rcutree_report_cpu_starting(). As a
  result it is guaranteed that further rcu_barrier() won't try to lock
  the nocb_lock for that target and thus won't risk an imbalance.

  Therefore barrier_mutex doesn't need to be locked anymore upon
  deoffloading.

* Assume the queue is empty before offloading, as
  rcutree_migrate_callbacks() took care of everything.

  This means that further calls to rcu_barrier() will find an empty
  queue until the CPU goes through rcutree_report_cpu_starting(). As a
  result it is guaranteed that further rcu_barrier() won't risk a
  nocb_lock imbalance.

  Therefore barrier_mutex doesn't need to be locked anymore upon
  offloading.

* No need to flush bypass anymore.

Further simplifications will follow in upcoming patches.

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>