hrtimer: Use irqsave/irqrestore around __run_hrtimer()

__run_hrtimer() is called with the hrtimer_cpu_base.lock held and
interrupts disabled. Before invoking the timer callback the base lock is
dropped, but interrupts stay disabled.

The upcoming support for softirq based hrtimers requires that interrupts
are enabled before the timer callback is invoked.

To avoid code duplication, take hrtimer_cpu_base.lock with
raw_spin_lock_irqsave(flags) at the call site and hand in the flags as
a parameter. So raw_spin_unlock_irqrestore() before the callback invocation
will either keep interrupts disabled in interrupt context or restore to
interrupt enabled state when called from softirq context.

Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Anna-Maria Gleixner <anna-maria@linutronix.de>
Cc: Christoph Hellwig <hch@lst.de>
Cc: John Stultz <john.stultz@linaro.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: keescook@chromium.org
Link: http://lkml.kernel.org/r/20171221104205.7269-26-anna-maria@linutronix.de


Signed-off-by: Ingo Molnar <mingo@kernel.org>