stop_machine: Use raw spinlocks

Use raw-locks in stop_machine() to allow locking in irq-off and
preempt-disabled regions on -RT. This also documents the possible locking
context in general.

[bigeasy: update patch description.]
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Link: https://lkml.kernel.org/r/20180423191635.6014-1-bigeasy@linutronix.de