thermal: avoid division by zero in power allocator

During boot I get a div by zero Oops regression starting in v4.3-rc3.
Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
Reviewed-by: Javi Merino <javi.merino@arm.com>
Cc: Zhang Rui <rui.zhang@intel.com>
Cc: Eduardo Valentin <edubezval@gmail.com>
Cc: Daniel Kurtz <djkurtz@chromium.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/thermal/power_allocator.c

@@ -144,6 +144,16 @@ static void estimate_pid_constants(struct thermal_zone_device *tz,
 		switch_on_temp = 0;
 
 	temperature_threshold = control_temp - switch_on_temp;
+	/*
+	 * estimate_pid_constants() tries to find appropriate default
+	 * values for thermal zones that don't provide them. If a
+	 * system integrator has configured a thermal zone with two
+	 * passive trip points at the same temperature, that person
+	 * hasn't put any effort to set up the thermal zone properly
+	 * so just give up.
+	 */
+	if (!temperature_threshold)
+		return;
 
 	if (!tz->tzp->k_po || force)
 		tz->tzp->k_po = int_to_frac(sustainable_power) /