hrtimer: Protect lockless access to timer->base

The update to timer->base is protected by the base->cpu_base->lock().
However, hrtimer_cancel_wait_running() does access it lockless.  So the
compiler is allowed to refetch timer->base which can cause havoc when the
timer base is changed concurrently.

Use READ_ONCE() to prevent this.

[ tglx: Adapted from a RT patch ]

Signed-off-by: Julien Grall <julien.grall@arm.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20190821092409.13225-2-julien.grall@arm.com