watchdog: Mark watchdog_hrtimer to expire in hard interrupt context

The watchdog hrtimer must expire in hard interrupt context even on
PREEMPT_RT=y kernels as otherwise the hard/softlockup detection logic would
not work.

No functional change.

[ tglx: Split out from larger combo patch. Added changelog ]

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20190726185753.262895510@linutronix.de

kernel/watchdog.c

@@ -490,10 +490,10 @@ static void watchdog_enable(unsigned int cpu)
 	 * Start the timer first to prevent the NMI watchdog triggering
 	 * before the timer has a chance to fire.
 	 */
-	hrtimer_init(hrtimer, CLOCK_MONOTONIC, HRTIMER_MODE_REL);
+	hrtimer_init(hrtimer, CLOCK_MONOTONIC, HRTIMER_MODE_REL_HARD);
 	hrtimer->function = watchdog_timer_fn;
 	hrtimer_start(hrtimer, ns_to_ktime(sample_period),
-		      HRTIMER_MODE_REL_PINNED);
+		      HRTIMER_MODE_REL_PINNED_HARD);
 
 	/* Initialize timestamp */
 	__touch_watchdog();