hrtimer: Prevent queuing of hrtimer without a function callback

The hrtimer function callback must not be NULL. It has to be specified by
the call side but it is not validated by the hrtimer code. When a hrtimer
is queued without a function callback, the kernel crashes with a null
pointer dereference when trying to execute the callback in __run_hrtimer().

Introduce a validation before queuing the hrtimer in
hrtimer_start_range_ns().

[anna-maria: Rephrase commit message]

Signed-off-by: Phil Chang <[e-mail]>
Signed-off-by: Anna-Maria Behnsen <[e-mail]>
Signed-off-by: Thomas Gleixner <[e-mail]>
Reviewed-by: Anna-Maria Behnsen <[e-mail]>

kernel/time/hrtimer.c

···
 	struct hrtimer_clock_base *base;
 	unsigned long flags;
 
+	if (WARN_ON_ONCE(!timer->function))
+		return;
 	/*
 	 * Check whether the HRTIMER_MODE_SOFT bit and hrtimer.is_soft
 	 * match on CONFIG_PREEMPT_RT = n. With PREEMPT_RT check the hard