leds: ledtrig-backlight: replace strict_strtoul() with kstrtoul()

The usage of strict_strtoul() is not preferred, because
strict_strtoul() is obsolete. Thus, kstrtoul() should be
used.
Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Bryan Wu <cooloney@gmail.com>

drivers/leds/ledtrig-backlight.c

@@ -76,7 +76,7 @@ static ssize_t bl_trig_invert_store(struct device *dev,
 	unsigned long invert;
 	int ret;
 
-	ret = strict_strtoul(buf, 10, &invert);
+	ret = kstrtoul(buf, 10, &invert);
 	if (ret < 0)
 		return ret;