cpu hotplug: Document why PREEMPT_RT uses a spinlock

The patch:

    cpu: Make hotplug.lock a "sleeping" spinlock on RT

    Tasks can block on hotplug.lock in pin_current_cpu(), but their
    state might be != RUNNING. So the mutex wakeup will set the state
    unconditionally to RUNNING. That might cause spurious unexpected
    wakeups. We could provide a state preserving mutex_lock() function,
    but this is semantically backwards. So instead we convert the
    hotplug.lock() to a spinlock for RT, which has the state preserving
    semantics already.

Fixed a bug where the hotplug lock on PREEMPT_RT can be called after a
task set its state to TASK_UNINTERRUPTIBLE and before it called
schedule. If the hotplug_lock used a mutex, and there was contention,
the current task's state would be turned to TASK_RUNNABLE and the
schedule call will not sleep. This caused unexpected results.

Although the patch had a description of the change, the code had no
comments about it. This causes confusion to those that review the code,
and as PREEMPT_RT is held in a quilt queue and not git, it's not as easy
to see why a change was made. Even if it was in git, the code should
still have a comment for something as subtle as this.

Document the rational for using a spinlock on PREEMPT_RT in the hotplug
lock code.

Reported-by: Nicholas Mc Guire <der.herr@hofr.at>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>

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